Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with no Invariant Sections, no Front-Cover texts, and
no Back-Cover Texts. A copy of the license is included in the section
entitled “GNU Free Documentation License.”

1 Introduction

A physicist, an engineer, and a computer scientist were discussing the

nature of God. “Surely a Physicist,” said the physicist, “because

early in the Creation, God made Light; and you know, Maxwell's

equations, the dual nature of electromagnetic waves, the relativistic

consequences...” “An Engineer!,” said the engineer, “because

before making Light, God split the Chaos into Land and Water; it takes a

hell of an engineer to handle that big amount of mud, and orderly

separation of solids from liquids...” The computer scientist

shouted: “And the Chaos, where do you think it was coming from, hmm?”

—Anonymous

Autoconf is a tool for producing shell scripts that automatically
configure software source code packages to adapt to many kinds of
Posix-like systems. The configuration scripts produced by Autoconf
are independent of Autoconf when they are run, so their users do not
need to have Autoconf.

The configuration scripts produced by Autoconf require no manual user
intervention when run; they do not normally even need an argument
specifying the system type. Instead, they individually test for the
presence of each feature that the software package they are for might need.
(Before each check, they print a one-line message stating what they are
checking for, so the user doesn't get too bored while waiting for the
script to finish.) As a result, they deal well with systems that are
hybrids or customized from the more common Posix variants. There is
no need to maintain files that list the features supported by each
release of each variant of Posix.

For each software package that Autoconf is used with, it creates a
configuration script from a template file that lists the system features
that the package needs or can use. After the shell code to recognize
and respond to a system feature has been written, Autoconf allows it to
be shared by many software packages that can use (or need) that feature.
If it later turns out that the shell code needs adjustment for some
reason, it needs to be changed in only one place; all of the
configuration scripts can be regenerated automatically to take advantage
of the updated code.

Those who do not understand Autoconf are condemned to reinvent it, poorly.
The primary goal of Autoconf is making the user's life easier;
making the maintainer's life easier is only a secondary goal.
Put another way, the primary goal is not to make the generation of
configure automatic for package maintainers (although patches
along that front are welcome, since package maintainers form the user
base of Autoconf); rather, the goal is to make configure
painless, portable, and predictable for the end user of each
autoconfiscated package. And to this degree, Autoconf is highly
successful at its goal — most complaints to the Autoconf list are
about difficulties in writing Autoconf input, and not in the behavior of
the resulting configure. Even packages that don't use Autoconf
will generally provide a configure script, and the most common
complaint about these alternative home-grown scripts is that they fail
to meet one or more of the GNU Coding Standards (see Configuration) that users
have come to expect from Autoconf-generated configure scripts.

The Metaconfig package is similar in purpose to Autoconf, but the
scripts it produces require manual user intervention, which is quite
inconvenient when configuring large source trees. Unlike Metaconfig
scripts, Autoconf scripts can support cross-compiling, if some care is
taken in writing them.

Autoconf does not solve all problems related to making portable
software packages—for a more complete solution, it should be used in
concert with other GNU build tools like Automake and
Libtool. These other tools take on jobs like the creation of a
portable, recursive makefile with all of the standard targets,
linking of shared libraries, and so on. See The GNU Build System,
for more information.

Autoconf requires GNU M4 version 1.4.6 or later in order to
generate the scripts. It uses features that some versions of M4,
including GNU M4 1.3, do not have. Autoconf works better
with GNU M4 version 1.4.14 or later, though this is not
required.

See Autoconf 1, for information about upgrading from version 1.
See History, for the story of Autoconf's development. See FAQ,
for answers to some common questions about Autoconf.

See the Autoconf web page for up-to-date information, details on the mailing
lists, pointers to a list of known bugs, etc.

If possible, first check that your bug is
not already solved in current development versions, and that it has not
been reported yet. Be sure to include all the needed information and a
short configure.ac that demonstrates the problem.

Because of its mission, the Autoconf package itself
includes only a set of often-used
macros that have already demonstrated their usefulness. Nevertheless,
if you wish to share your macros, or find existing ones, see the
Autoconf Macro Archive, which is kindly run by Peter Simons.

2 The GNU Build System

Autoconf solves an important problem—reliable discovery of
system-specific build and runtime information—but this is only one
piece of the puzzle for the development of portable software. To this
end, the GNU project has developed a suite of integrated
utilities to finish the job Autoconf started: the GNU build
system, whose most important components are Autoconf, Automake, and
Libtool. In this chapter, we introduce you to those tools, point you
to sources of more information, and try to convince you to use the
entire GNU build system for your software.

2.1 Automake

The ubiquity of make means that a makefile is almost the
only viable way to distribute automatic build rules for software, but
one quickly runs into its numerous limitations. Its lack of
support for automatic dependency tracking, recursive builds in
subdirectories, reliable timestamps (e.g., for network file systems), and
so on, mean that developers must painfully (and often incorrectly)
reinvent the wheel for each project. Portability is non-trivial, thanks
to the quirks of make on many systems. On top of all this is the
manual labor required to implement the many standard targets that users
have come to expect (make install, make distclean,
make uninstall, etc.). Since you are, of course, using Autoconf,
you also have to insert repetitive code in your Makefile.in to
recognize @CC@, @CFLAGS@, and other substitutions
provided by configure. Into this mess steps Automake.
Automake allows you to specify your build needs in a Makefile.am
file with a vastly simpler and more powerful syntax than that of a plain
makefile, and then generates a portable Makefile.in for
use with Autoconf. For example, the Makefile.am to build and
install a simple “Hello world” program might look like:

bin_PROGRAMS = hello
hello_SOURCES = hello.c

The resulting Makefile.in (~400 lines) automatically supports all
the standard targets, the substitutions provided by Autoconf, automatic
dependency tracking, VPATH building, and so on. make
builds the hello program, and make install installs it
in /usr/local/bin (or whatever prefix was given to
configure, if not /usr/local).

The benefits of Automake increase for larger packages (especially ones
with subdirectories), but even for small programs the added convenience
and portability can be substantial. And that's not all...

2.2 Gnulib

GNU software has a well-deserved reputation for running on
many different types of systems. While our primary goal is to write
software for the GNU system, many users and developers have
been introduced to us through the systems that they were already using.

Gnulib is a central location for common GNU code, intended to
be shared among free software packages. Its components are typically
shared at the source level, rather than being a library that gets built,
installed, and linked against. The idea is to copy files from Gnulib
into your own source tree. There is no distribution tarball; developers
should just grab source modules from the repository. The source files
are available online, under various licenses, mostly GNU
GPL or GNU LGPL.

Gnulib modules typically contain C source code along with Autoconf
macros used to configure the source code. For example, the Gnulib
stdbool module implements a stdbool.h header that nearly
conforms to C99, even on old-fashioned hosts that lack stdbool.h.
This module contains a source file for the replacement header, along
with an Autoconf macro that arranges to use the replacement header on
old-fashioned systems.

2.3 Libtool

Often, one wants to build not only programs, but libraries, so that
other programs can benefit from the fruits of your labor. Ideally, one
would like to produce shared (dynamically linked) libraries,
which can be used by multiple programs without duplication on disk or in
memory and can be updated independently of the linked programs.
Producing shared libraries portably, however, is the stuff of
nightmares—each system has its own incompatible tools, compiler flags,
and magic incantations. Fortunately, GNU provides a solution:
Libtool.
Libtool handles all the requirements of building shared libraries for
you, and at this time seems to be the only way to do so with any
portability. It also handles many other headaches, such as: the
interaction of Make rules with the variable suffixes of
shared libraries, linking reliably with shared libraries before they are
installed by the superuser, and supplying a consistent versioning system
(so that different versions of a library can be installed or upgraded
without breaking binary compatibility). Although Libtool, like
Autoconf, can be used without Automake, it is most simply utilized in
conjunction with Automake—there, Libtool is used automatically
whenever shared libraries are needed, and you need not know its syntax.

2.4 Pointers

Developers who are used to the simplicity of make for small
projects on a single system might be daunted at the prospect of
learning to use Automake and Autoconf. As your software is
distributed to more and more users, however, you otherwise
quickly find yourself putting lots of effort into reinventing the
services that the GNU build tools provide, and making the
same mistakes that they once made and overcame. (Besides, since
you're already learning Autoconf, Automake is a piece of cake.)

There are a number of places that you can go to for more information on
the GNU build tools.

3 Making configure Scripts

The configuration scripts that Autoconf produces are by convention
called configure. When run, configure creates several
files, replacing configuration parameters in them with appropriate
values. The files that configure creates are:

an optional shell script normally called config.cache
(created when using ‘configure --config-cache’) that
saves the results of running many of the tests (see Cache Files);

a file called config.log containing any messages produced by
compilers, to help debugging if configure makes a mistake.

To create a configure script with Autoconf, you need to write an
Autoconf input file configure.ac (or configure.in) and run
autoconf on it. If you write your own feature tests to
supplement those that come with Autoconf, you might also write files
called aclocal.m4 and acsite.m4. If you use a C header
file to contain #define directives, you might also run
autoheader, and you can distribute the generated file
config.h.in with the package.

Here is a diagram showing how the files that can be used in
configuration are produced. Programs that are executed are suffixed by
‘*’. Optional files are enclosed in square brackets (‘[]’).
autoconf and autoheader also read the installed Autoconf
macro files (by reading autoconf.m4).

Files used in preparing a software package for distribution, when using
just Autoconf:

3.1 Writing configure.ac

To produce a configure script for a software package, create a
file called configure.ac that contains invocations of the
Autoconf macros that test the system features your package needs or can
use. Autoconf macros already exist to check for many features; see
Existing Tests, for their descriptions. For most other features,
you can use Autoconf template macros to produce custom checks; see
Writing Tests, for information about them. For especially tricky
or specialized features, configure.ac might need to contain some
hand-crafted shell commands; see Portable Shell Programming. The autoscan program can give you a good start
in writing configure.ac (see autoscan Invocation, for more
information).

Previous versions of Autoconf promoted the name configure.in,
which is somewhat ambiguous (the tool needed to process this file is not
described by its extension), and introduces a slight confusion with
config.h.in and so on (for which ‘.in’ means “to be
processed by configure”). Using configure.ac is now
preferred.

3.1.1 A Shell Script Compiler

Just as for any other computer language, in order to properly program
configure.ac in Autoconf you must understand what problem
the language tries to address and how it does so.

The problem Autoconf addresses is that the world is a mess. After all,
you are using Autoconf in order to have your package compile easily on
all sorts of different systems, some of them being extremely hostile.
Autoconf itself bears the price for these differences: configure
must run on all those systems, and thus configure must limit itself
to their lowest common denominator of features.

Naturally, you might then think of shell scripts; who needs
autoconf? A set of properly written shell functions is enough to
make it easy to write configure scripts by hand. Sigh!
Unfortunately, even in 2008, where shells without any function support are
far and few between, there are pitfalls to avoid when making use of them.
Also, finding a Bourne shell that accepts shell functions is not trivial,
even though there is almost always one on interesting porting targets.

So, what is really needed is some kind of compiler, autoconf,
that takes an Autoconf program, configure.ac, and transforms it
into a portable shell script, configure.

How does autoconf perform this task?

There are two obvious possibilities: creating a brand new language or
extending an existing one. The former option is attractive: all
sorts of optimizations could easily be implemented in the compiler and
many rigorous checks could be performed on the Autoconf program
(e.g., rejecting any non-portable construct). Alternatively, you can
extend an existing language, such as the sh (Bourne shell)
language.

Autoconf does the latter: it is a layer on top of sh. It was
therefore most convenient to implement autoconf as a macro
expander: a program that repeatedly performs macro expansions on
text input, replacing macro calls with macro bodies and producing a pure
sh script in the end. Instead of implementing a dedicated
Autoconf macro expander, it is natural to use an existing
general-purpose macro language, such as M4, and implement the extensions
as a set of M4 macros.

3.1.2 The Autoconf Language

The Autoconf language differs from many other computer
languages because it treats actual code the same as plain text. Whereas
in C, for instance, data and instructions have different syntactic
status, in Autoconf their status is rigorously the same. Therefore, we
need a means to distinguish literal strings from text to be expanded:
quotation.

When calling macros that take arguments, there must not be any white
space between the macro name and the open parenthesis.

AC_INIT ([oops], [1.0]) # incorrect
AC_INIT([hello], [1.0]) # good

Arguments should
be enclosed within the quote characters ‘[’ and ‘]’, and be
separated by commas. Any leading blanks or newlines in arguments are ignored,
unless they are quoted. You should always quote an argument that
might contain a macro name, comma, parenthesis, or a leading blank or
newline. This rule applies recursively for every macro
call, including macros called from other macros. For more details on
quoting rules, see Programming in M4.

For instance:

AC_CHECK_HEADER([stdio.h],
[AC_DEFINE([HAVE_STDIO_H], [1],
[Define to 1 if you have <stdio.h>.])],
[AC_MSG_ERROR([sorry, can't do anything for you])])

is quoted properly. You may safely simplify its quotation to:

AC_CHECK_HEADER([stdio.h],
[AC_DEFINE([HAVE_STDIO_H], 1,
[Define to 1 if you have <stdio.h>.])],
[AC_MSG_ERROR([sorry, can't do anything for you])])

because ‘1’ cannot contain a macro call. Here, the argument of
AC_MSG_ERROR must be quoted; otherwise, its comma would be
interpreted as an argument separator. Also, the second and third arguments
of ‘AC_CHECK_HEADER’ must be quoted, since they contain
macro calls. The three arguments ‘HAVE_STDIO_H’, ‘stdio.h’,
and ‘Define to 1 if you have <stdio.h>.’ do not need quoting, but
if you unwisely defined a macro with a name like ‘Define’ or
‘stdio’ then they would need quoting. Cautious Autoconf users
would keep the quotes, but many Autoconf users find such precautions
annoying, and would rewrite the example as follows:

AC_CHECK_HEADER(stdio.h,
[AC_DEFINE(HAVE_STDIO_H, 1,
[Define to 1 if you have <stdio.h>.])],
[AC_MSG_ERROR([sorry, can't do anything for you])])

This is safe, so long as you adopt good naming conventions and do not
define macros with names like ‘HAVE_STDIO_H’, ‘stdio’, or
‘h’. Though it is also safe here to omit the quotes around
‘Define to 1 if you have <stdio.h>.’ this is not recommended, as
message strings are more likely to inadvertently contain commas.

The following example is wrong and dangerous, as it is underquoted:

AC_CHECK_HEADER(stdio.h,
AC_DEFINE(HAVE_STDIO_H, 1,
Define to 1 if you have <stdio.h>.),
AC_MSG_ERROR([sorry, can't do anything for you]))

In other cases, you may have to use text that also resembles a macro
call. You must quote that text even when it is not passed as a macro
argument. For example, these two approaches in configure.ac
(quoting just the potential problems, or quoting the entire line) will
protect your script in case autoconf ever adds a macro AC_DC:

When you use the same text in a macro argument, you must therefore have
an extra quotation level (since one is stripped away by the macro
substitution). In general, then, it is a good idea to use double
quoting for all literal string arguments, either around just the
problematic portions, or over the entire argument:

However, the above example triggers a warning about a possibly
unexpanded macro when running autoconf, because it collides
with the namespace of macros reserved for the Autoconf language. To be
really safe, you can use additional escaping (either a quadrigraph, or
creative shell constructs) to silence that particular warning:

You are now able to understand one of the constructs of Autoconf that
has been continually misunderstood... The rule of thumb is that
whenever you expect macro expansion, expect quote expansion;
i.e., expect one level of quotes to be lost. For instance:

is incorrect: here, the first argument of AC_LANG_SOURCE is
‘char b[10];’ and is expanded once, which results in
‘char b10;’; and the AC_LANG_SOURCE is also expanded prior
to being passed to AC_COMPILE_IFELSE. (There was an idiom common
in Autoconf's past to
address this issue via the M4 changequote primitive, but do not
use it!) Let's take a closer look: the author meant the first argument
to be understood as a literal, and therefore it must be quoted twice;
likewise, the intermediate AC_LANG_SOURCE macro should be quoted
once so that it is only expanded after the rest of the body of
AC_COMPILE_IFELSE is in place:

On the other hand, descriptions (e.g., the last parameter of
AC_DEFINE or AS_HELP_STRING) are not literals—they
are subject to line breaking, for example—and should not be double quoted.
Even if these descriptions are short and are not actually broken, double
quoting them yields weird results.

Some macros take optional arguments, which this documentation represents
as [arg] (not to be confused with the quote characters). You may
just leave them empty, or use ‘[]’ to make the emptiness of the
argument explicit, or you may simply omit the trailing commas. The
three lines below are equivalent:

It is best to put each macro call on its own line in
configure.ac. Most of the macros don't add extra newlines; they
rely on the newline after the macro call to terminate the commands.
This approach makes the generated configure script a little
easier to read by not inserting lots of blank lines. It is generally
safe to set shell variables on the same line as a macro call, because
the shell allows assignments without intervening newlines.

You can include comments in configure.ac files by starting them
with the ‘#’. For example, it is helpful to begin
configure.ac files with a line like this:

3.1.3 Standard configure.ac Layout

The order in which configure.ac calls the Autoconf macros is not
important, with a few exceptions. Every configure.ac must
contain a call to AC_INIT before the checks, and a call to
AC_OUTPUT at the end (see Output). Additionally, some macros
rely on other macros having been called first, because they check
previously set values of some variables to decide what to do. These
macros are noted in the individual descriptions (see Existing Tests), and they also warn you when configure is created if they
are called out of order.

To encourage consistency, here is a suggested order for calling the
Autoconf macros. Generally speaking, the things near the end of this
list are those that could depend on things earlier in it. For example,
library functions could be affected by types and libraries.

3.2 Using autoscan to Create configure.ac

The autoscan program can help you create and/or maintain a
configure.ac file for a software package. autoscan
examines source files in the directory tree rooted at a directory given
as a command line argument, or the current directory if none is given.
It searches the source files for common portability problems and creates
a file configure.scan which is a preliminary configure.ac
for that package, and checks a possibly existing configure.ac for
completeness.

When using autoscan to create a configure.ac, you
should manually examine configure.scan before renaming it to
configure.ac; it probably needs some adjustments.
Occasionally, autoscan outputs a macro in the wrong order
relative to another macro, so that autoconf produces a warning;
you need to move such macros manually. Also, if you want the package to
use a configuration header file, you must add a call to
AC_CONFIG_HEADERS (see Configuration Headers). You might
also have to change or add some #if directives to your program in
order to make it work with Autoconf (see ifnames Invocation, for
information about a program that can help with that job).

When using autoscan to maintain a configure.ac, simply
consider adding its suggestions. The file autoscan.log
contains detailed information on why a macro is requested.

autoscan uses several data files (installed along with Autoconf)
to determine which macros to output when it finds particular symbols in
a package's source files. These data files all have the same format:
each line consists of a symbol, one or more blanks, and the Autoconf macro to
output if that symbol is encountered. Lines starting with ‘#’ are
comments.

autoscan accepts the following options:

--help

-h

Print a summary of the command line options and exit.

--version

-V

Print the version number of Autoconf and exit.

--verbose

-v

Print the names of the files it examines and the potentially interesting
symbols it finds in them. This output can be voluminous.

3.3 Using ifnames to List Conditionals

ifnames can help you write configure.ac for a software
package. It prints the identifiers that the package already uses in C
preprocessor conditionals. If a package has already been set up to have
some portability, ifnames can thus help you figure out what its
configure needs to check for. It may help fill in some gaps in a
configure.ac generated by autoscan (see autoscan Invocation).

ifnames scans all of the C source files named on the command line
(or the standard input, if none are given) and writes to the standard
output a sorted list of all the identifiers that appear in those files
in #if, #elif, #ifdef, or #ifndef
directives. It prints each identifier on a line, followed by a
space-separated list of the files in which that identifier occurs.

3.4 Using autoconf to Create configure

To create configure from configure.ac, run the
autoconf program with no arguments. autoconf processes
configure.ac with the M4 macro processor, using the
Autoconf macros. If you give autoconf an argument, it reads that
file instead of configure.ac and writes the configuration script
to the standard output instead of to configure. If you give
autoconf the argument -, it reads from the standard
input instead of configure.ac and writes the configuration script
to the standard output.

The Autoconf macros are defined in several files. Some of the files are
distributed with Autoconf; autoconf reads them first. Then it
looks for the optional file acsite.m4 in the directory that
contains the distributed Autoconf macro files, and for the optional file
aclocal.m4 in the current directory. Those files can contain
your site's or the package's own Autoconf macro definitions
(see Writing Autoconf Macros, for more information). If a macro is
defined in more than one of the files that autoconf reads, the
last definition it reads overrides the earlier ones.

autoconf accepts the following options:

--help

-h

Print a summary of the command line options and exit.

--version

-V

Print the version number of Autoconf and exit.

--verbose

-v

Report processing steps.

--debug

-d

Don't remove the temporary files.

--force

-f

Remake configure even if newer than its input files.

--include=dir

-I dir

Append dir to the include path. Multiple invocations accumulate.

--prepend-include=dir

-B dir

Prepend dir to the include path. Multiple invocations accumulate.

--output=file

-o file

Save output (script or trace) to file. The file - stands
for the standard output.

--warnings=category

-W category

Report the warnings related to category (which can actually be a
comma separated list). See Reporting Messages, macro
AC_DIAGNOSE, for a comprehensive list of categories. Special
values include:

‘all’

report all the warnings

‘none’

report none

‘error’

treats warnings as errors

‘no-category’

disable warnings falling into category

Warnings about ‘syntax’ are enabled by default, and the environment
variable WARNINGS, a comma separated list of categories, is
honored as well. Passing -W category actually behaves as if
you had passed --warnings syntax,$WARNINGS,category. To
disable the defaults and WARNINGS, and then
enable warnings about obsolete constructs, use -W
none,obsolete.

Because autoconf uses autom4te behind the scenes, it
displays a back trace for errors, but not for warnings; if you want
them, just pass -W error. See autom4te Invocation, for some
examples.

--trace=macro[:format]

-t macro[:format]

Do not create the configure script, but list the calls to
macro according to the format. Multiple --trace
arguments can be used to list several macros. Multiple --trace
arguments for a single macro are not cumulative; instead, you should
just make format as long as needed.

The format is a regular string, with newlines if desired, and
several special escape codes. It defaults to ‘$f:$l:$n:$%’; see
autom4te Invocation, for details on the format.

--initialization

-i

By default, --trace does not trace the initialization of the
Autoconf macros (typically the AC_DEFUN definitions). This
results in a noticeable speedup, but can be disabled by this option.

It is often necessary to check the content of a configure.ac
file, but parsing it yourself is extremely fragile and error-prone. It
is suggested that you rely upon --trace to scan
configure.ac. For instance, to find the list of variables that
are substituted, use:

3.5 Using autoreconf to Update configure Scripts

Installing the various components of the GNU Build System can be
tedious: running autopoint for Gettext, automake for
Makefile.in etc. in each directory. It may be needed either
because some tools such as automake have been updated on your
system, or because some of the sources such as configure.ac have
been updated, or finally, simply in order to install the GNU Build
System in a fresh tree.

autoreconf runs autoconf, autoheader,
aclocal, automake, libtoolize, and
autopoint (when appropriate) repeatedly to update the
GNU Build System in the specified directories and their
subdirectories (see Subdirectories). By default, it only remakes
those files that are older than their sources. The environment variables
AUTOM4TE, AUTOCONF, AUTOHEADER, AUTOMAKE,
ACLOCAL, AUTOPOINT, LIBTOOLIZE, M4, and MAKE
may be used to override the invocation of the respective tools.

If you install a new version of some tool, you can make
autoreconf remake all of the files by giving it the
--force option.

See Automatic Remaking, for Make rules to automatically
rebuild configure scripts when their source files change. That
method handles the timestamps of configuration header templates
properly, but does not pass --autoconf-dir=dir or
--localdir=dir.

Gettext supplies the autopoint command to add translation
infrastructure to a source package. If you use autopoint,
your configure.ac should invoke both AM_GNU_GETTEXT and
AM_GNU_GETTEXT_VERSION(gettext-version). See Invoking the autopoint Program, for further details.

autoreconf accepts the following options:

--help

-h

Print a summary of the command line options and exit.

--version

-V

Print the version number of Autoconf and exit.

--verbose

-v

Print the name of each directory autoreconf examines and the
commands it runs. If given two or more times, pass --verbose
to subordinate tools that support it.

--debug

-d

Don't remove the temporary files.

--force

-f

Remake even configure scripts and configuration headers that are
newer than their input files (configure.ac and, if present,
aclocal.m4).

--install

-i

Install the missing auxiliary files in the package. By default, files
are copied; this can be changed with --symlink.

Do not rebuild files in subdirectories to configure (see Subdirectories,
macro AC_CONFIG_SUBDIRS).

--symlink

-s

When used with --install, install symbolic links to the missing
auxiliary files instead of copying them.

--make

-m

When the directories were configured, update the configuration by
running ‘./config.status --recheck && ./config.status’, and then
run ‘make’.

--include=dir

-I dir

Append dir to the include path. Multiple invocations accumulate.
Passed on to aclocal, autoconf and
autoheader internally.

--prepend-include=dir

-B dir

Prepend dir to the include path. Multiple invocations accumulate.
Passed on to autoconf and autoheader internally.

--warnings=category

-W category

Report the warnings related to category (which can actually be a
comma separated list).

‘cross’

related to cross compilation issues.

‘obsolete’

report the uses of obsolete constructs.

‘portability’

portability issues

‘syntax’

dubious syntactic constructs.

‘all’

report all the warnings

‘none’

report none

‘error’

treats warnings as errors

‘no-category’

disable warnings falling into category

Warnings about ‘syntax’ are enabled by default, and the environment
variable WARNINGS, a comma separated list of categories, is
honored as well. Passing -W category actually behaves as if
you had passed --warnings syntax,$WARNINGS,category. To
disable the defaults and WARNINGS, and then
enable warnings about obsolete constructs, use -W
none,obsolete.

If you want autoreconf to pass flags that are not listed here
on to aclocal, set ACLOCAL_AMFLAGS in your Makefile.am.
Due to a limitation in the Autoconf implementation these flags currently
must be set on a single line in Makefile.am, without any
backslash-newlines.

4 Initialization and Output Files

Autoconf-generated configure scripts need some information about
how to initialize, such as how to find the package's source files and
about the output files to produce. The following sections describe the
initialization and the creation of output files.

4.1 Initializing configure

Every configure script must call AC_INIT before doing
anything else that produces output. Calls to silent macros, such as
AC_DEFUN, may also occur prior to AC_INIT, although these
are generally used via aclocal.m4, since that is implicitly
included before the start of configure.ac. The only other
required macro is AC_OUTPUT (see Output).

— Macro: AC_INIT (package, version, [bug-report], [tarname], [url])

Process any command-line arguments and perform initialization
and verification.

Set the name of the package and its version. These are
typically used in --version support, including that of
configure. The optional argument bug-report should be
the email to which users should send bug reports. The package
tarname differs from package: the latter designates the full
package name (e.g., ‘GNU Autoconf’), while the former is meant for
distribution tar ball names (e.g., ‘autoconf’). It defaults to
package with ‘GNU ’ stripped, lower-cased, and all characters
other than alphanumerics and underscores are changed to ‘-’. If
provided, url should be the home page for the package.

The arguments of AC_INIT must be static, i.e., there should not
be any shell computation, quotes, or newlines, but they can be computed
by M4. This is because the package information strings are expanded at
M4 time into several contexts, and must give the same text at shell time
whether used in single-quoted strings, double-quoted strings, quoted
here-documents, or unquoted here-documents. It is permissible to use
m4_esyscmd or m4_esyscmd_s for computing a version string
that changes with every commit to a version control system (in fact,
Autoconf does just that, for all builds of the development tree made
between releases).

Exactly bug-report, if one was provided. Typically an email
address, or URL to a bug management web page.

AC_PACKAGE_URL, PACKAGE_URL

Exactly url, if one was provided. If url was empty, but
package begins with ‘GNU ’, then this defaults to
‘http://www.gnu.org/software/tarname/’, otherwise, no URL is
assumed.

If your configure script does its own option processing, it
should inspect ‘$@’ or ‘$*’ immediately after calling
AC_INIT, because other Autoconf macros liberally use the
set command to process strings, and this has the side effect
of updating ‘$@’ and ‘$*’. However, we suggest that you use
standard macros like AC_ARG_ENABLE instead of attempting to
implement your own option processing. See Site Configuration.

4.2 Dealing with Autoconf versions

The following optional macros can be used to help choose the minimum
version of Autoconf that can successfully compile a given
configure.ac.

— Macro: AC_PREREQ (version)

Ensure that a recent enough version of Autoconf is being used. If the
version of Autoconf being used to create configure is
earlier than version, print an error message to the standard
error output and exit with failure (exit status is 63). For example:

AC_PREREQ([2.69])

This macro may be used before AC_INIT.

— Macro: AC_AUTOCONF_VERSION

This macro was introduced in Autoconf 2.62. It identifies the version
of Autoconf that is currently parsing the input file, in a format
suitable for m4_version_compare (see m4_version_compare); in
other words, for this release of Autoconf, its value is
‘2.69’. One potential use of this macro is for writing
conditional fallbacks based on when a feature was added to Autoconf,
rather than using AC_PREREQ to require the newer version of
Autoconf. However, remember that the Autoconf philosophy favors feature
checks over version checks.

You should not expand this macro directly; use
‘m4_defn([AC_AUTOCONF_VERSION])’ instead. This is because some
users might
have a beta version of Autoconf installed, with arbitrary letters
included in its version string. This means it is possible for the
version string to contain the name of a defined macro, such that
expanding AC_AUTOCONF_VERSION would trigger the expansion of that
macro during rescanning, and change the version string to be different
than what you intended to check.

4.3 Notices in configure

The following macros manage version numbers for configure
scripts. Using them is optional.

— Macro: AC_COPYRIGHT (copyright-notice)

State that, in addition to the Free Software Foundation's copyright on
the Autoconf macros, parts of your configure are covered by the
copyright-notice.

The copyright-notice shows up in both the head of
configure and in ‘configure --version’.

— Macro: AC_REVISION (revision-info)

Copy revision stamp revision-info into the configure
script, with any dollar signs or double-quotes removed. This macro lets
you put a revision stamp from configure.ac into configure
without RCS or CVS changing it when you check in
configure. That way, you can determine easily which revision of
configure.ac a particular configure corresponds to.

4.4 Finding configure Input

— Macro: AC_CONFIG_SRCDIR (unique-file-in-source-dir)

unique-file-in-source-dir is some file that is in the package's
source directory; configure checks for this file's existence to
make sure that the directory that it is told contains the source code in
fact does. Occasionally people accidentally specify the wrong directory
with --srcdir; this is a safety check. See configure Invocation, for more information.

Packages that do manual configuration or use the install program
might need to tell configure where to find some other shell
scripts by calling AC_CONFIG_AUX_DIR, though the default places
it looks are correct for most cases.

— Macro: AC_CONFIG_AUX_DIR (dir)

Use the auxiliary build tools (e.g., install-sh,
config.sub, config.guess, Cygnus configure,
Automake and Libtool scripts, etc.) that are in directory dir.
These are auxiliary files used in configuration. dir can be
either absolute or relative to srcdir. The default is
srcdir or srcdir/.. or
srcdir/../.., whichever is the first that contains
install-sh. The other files are not checked for, so that using
AC_PROG_INSTALL does not automatically require distributing the
other auxiliary files. It checks for install.sh also, but that
name is obsolete because some make have a rule that creates
install from it if there is no makefile.

The auxiliary directory is commonly named build-aux.
If you need portability to DOS variants, do not name the
auxiliary directory aux. See File System Conventions.

— Macro: AC_REQUIRE_AUX_FILE (file)

Declares that file is expected in the directory defined above. In
Autoconf proper, this macro does nothing: its sole purpose is to be
traced by third-party tools to produce a list of expected auxiliary
files. For instance it is called by macros like AC_PROG_INSTALL
(see Particular Programs) or AC_CANONICAL_BUILD
(see Canonicalizing) to register the auxiliary files they need.

Similarly, packages that use aclocal should declare where
local macros can be found using AC_CONFIG_MACRO_DIR.

— Macro: AC_CONFIG_MACRO_DIR (dir)

Specify dir as the location of additional local Autoconf macros.
This macro is intended for use by future versions of commands like
autoreconf that trace macro calls. It should be called
directly from configure.ac so that tools that install macros for
aclocal can find the macros' declarations.

Note that if you use aclocal from Automake to generate
aclocal.m4, you must also set ACLOCAL_AMFLAGS = -I
dir in your top-level Makefile.am. Due to a limitation in
the Autoconf implementation of autoreconf, these include
directives currently must be set on a single line in Makefile.am,
without any backslash-newlines.

4.5 Outputting Files

Every Autoconf script, e.g., configure.ac, should finish by
calling AC_OUTPUT. That is the macro that generates and runs
config.status, which in turn creates the makefiles and any
other files resulting from configuration. This is the only required
macro besides AC_INIT (see Input).

— Macro: AC_OUTPUT

Generate config.status and launch it. Call this macro once, at
the end of configure.ac.

The location of your AC_OUTPUT invocation is the exact point
where configuration actions are taken: any code afterwards is
executed by configure once config.status was run. If
you want to bind actions to config.status itself
(independently of whether configure is being run), see
Running Arbitrary Configuration Commands.

Historically, the usage of AC_OUTPUT was somewhat different.
See Obsolete Macros, for a description of the arguments that
AC_OUTPUT used to support.

If you run make in subdirectories, you should run it using the
make variable MAKE. Most versions of make set
MAKE to the name of the make program plus any options it
was given. (But many do not include in it the values of any variables
set on the command line, so those are not passed on automatically.)
Some old versions of make do not set this variable. The
following macro allows you to use it even with those versions.

— Macro: AC_PROG_MAKE_SET

If the Make command, $MAKE if set or else ‘make’, predefines
$(MAKE), define output variable SET_MAKE to be empty.
Otherwise, define SET_MAKE to a macro definition that sets
$(MAKE), such as ‘MAKE=make’. Calls AC_SUBST for
SET_MAKE.

If you use this macro, place a line like this in each Makefile.in
that runs MAKE on other directories:

4.6 Performing Configuration Actions

configure is designed so that it appears to do everything itself,
but there is actually a hidden slave: config.status.
configure is in charge of examining your system, but it is
config.status that actually takes the proper actions based on the
results of configure. The most typical task of
config.status is to instantiate files.

This section describes the common behavior of the four standard
instantiating macros: AC_CONFIG_FILES, AC_CONFIG_HEADERS,
AC_CONFIG_COMMANDS and AC_CONFIG_LINKS. They all
have this prototype:

AC_CONFIG_ITEMS(tag..., [commands], [init-cmds])

where the arguments are:

tag...

A blank-or-newline-separated list of tags, which are typically the names of
the files to instantiate.

You are encouraged to use literals as tags. In particular, you
should avoid

The macros AC_CONFIG_FILES and AC_CONFIG_HEADERS use
special tag values: they may have the form ‘output’ or
‘output:inputs’. The file output is instantiated
from its templates, inputs (defaulting to ‘output.in’).

‘AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk])’,
for example, asks for
the creation of the file Makefile that contains the expansion of the
output variables in the concatenation of boiler/top.mk and
boiler/bot.mk.

The special value ‘-’ might be used to denote the standard output
when used in output, or the standard input when used in the
inputs. You most probably don't need to use this in
configure.ac, but it is convenient when using the command line
interface of ./config.status, see config.status Invocation,
for more details.

The inputs may be absolute or relative file names. In the latter
case they are first looked for in the build tree, and then in the source
tree. Input files should be text files, and a line length below 2000
bytes should be safe.

commands

Shell commands output literally into config.status, and
associated with a tag that the user can use to tell config.status
which commands to run. The commands are run each time a tag
request is given to config.status, typically each time the file
tag is created.

The variables set during the execution of configure are
not available here: you first need to set them via the
init-cmds. Nonetheless the following variables are precomputed:

srcdir

The name of the top source directory, assuming that the working
directory is the top build directory. This
is what the configure option --srcdir sets.

ac_top_srcdir

The name of the top source directory, assuming that the working
directory is the current build directory.

ac_top_build_prefix

The name of the top build directory, assuming that the working
directory is the current build directory.
It can be empty, or else ends with a slash, so that you may concatenate
it.

ac_srcdir

The name of the corresponding source directory, assuming that the
working directory is the current build directory.

tmp

The name of a temporary directory within the build tree, which you
can use if you need to create additional temporary files. The
directory is cleaned up when config.status is done or
interrupted. Please use package-specific file name prefixes to
avoid clashing with files that config.status may use
internally.

The current directory refers to the directory (or
pseudo-directory) containing the input part of tags. For
instance, running

Shell commands output unquoted near the beginning of
config.status, and executed each time config.status runs
(regardless of the tag). Because they are unquoted, for example,
‘$var’ is output as the value of var. init-cmds
is typically used by configure to give config.status some
variables it needs to run the commands.

You should be extremely cautious in your variable names: all the
init-cmds share the same name space and may overwrite each other
in unpredictable ways. Sorry...

All these macros can be called multiple times, with different
tag values, of course!

4.7 Creating Configuration Files

Make AC_OUTPUT create each file by copying an input
file (by default file.in), substituting the output variable
values.
This macro is one of the instantiating macros; see Configuration Actions. See Makefile Substitutions, for more information on using
output variables. See Setting Output Variables, for more information
on creating them. This macro creates the directory that the file is in
if it doesn't exist. Usually, makefiles are created this way,
but other files, such as .gdbinit, can be specified as well.

4.8 Substitutions in Makefiles

Each subdirectory in a distribution that contains something to be
compiled or installed should come with a file Makefile.in, from
which configure creates a file Makefile in that directory.
To create Makefile, configure performs a simple variable
substitution, replacing occurrences of ‘@variable@’ in
Makefile.in with the value that configure has determined
for that variable. Variables that are substituted into output files in
this way are called output variables. They are ordinary shell
variables that are set in configure. To make configure
substitute a particular variable into the output files, the macro
AC_SUBST must be called with that variable name as an argument.
Any occurrences of ‘@variable@’ for other variables are
left unchanged. See Setting Output Variables, for more information
on creating output variables with AC_SUBST.

A software package that uses a configure script should be
distributed with a file Makefile.in, but no makefile; that
way, the user has to properly configure the package for the local system
before compiling it.

4.8.1 Preset Output Variables

Some output variables are preset by the Autoconf macros. Some of the
Autoconf macros set additional output variables, which are mentioned in
the descriptions for those macros. See Output Variable Index, for a
complete list of output variables. See Installation Directory Variables, for the list of the preset ones related to installation
directories. Below are listed the other preset ones, many of which are
precious variables (see Setting Output Variables,
AC_ARG_VAR).

The preset variables which are available during config.status
(see Configuration Actions) may also be used during
configure tests. For example, it is permissible to reference
‘$srcdir’ when constructing a list of directories to pass via
option -I during a compiler feature check. When used in this
manner, coupled with the fact that configure is always run
from the top build directory, it is sufficient to use just
‘$srcdir’ instead of ‘$top_srcdir’.

— Variable: CFLAGS

Debugging and optimization options for the C compiler. If it is not set
in the environment when configure runs, the default value is set
when you call AC_PROG_CC (or empty if you don't). configure
uses this variable when compiling or linking programs to test for C features.

If a compiler option affects only the behavior of the preprocessor
(e.g., -Dname), it should be put into CPPFLAGS
instead. If it affects only the linker (e.g., -Ldirectory),
it should be put into LDFLAGS instead. If it
affects only the compiler proper, CFLAGS is the natural home for
it. If an option affects multiple phases of the compiler, though,
matters get tricky. One approach to put such options directly into
CC, e.g., CC='gcc -m64'. Another is to put them into both
CPPFLAGS and LDFLAGS, but not into CFLAGS.

However, remember that some Makefile variables are reserved by
the GNU Coding Standards for the use of the “user”—the person
building the package. For instance, CFLAGS is one such variable.

Sometimes package developers are tempted to set user variables such as
CFLAGS because it appears to make their job easier. However, the
package itself should never set a user variable, particularly not to
include switches that are required for proper compilation of the
package. Since these variables are documented as being for the package
builder, that person rightfully expects to be able to override any of
these variables at build time. If the package developer needs to add
switches without interfering with the user, the proper way to do that is
to introduce an additional variable. Automake makes this easy by
introducing AM_CFLAGS (see Flag Variables Ordering), but the concept is the same even if
Automake is not used.

— Variable: configure_input

A comment saying that the file was generated automatically by
configure and giving the name of the input file.
AC_OUTPUT adds a comment line containing this variable to the top
of every makefile it creates. For other files, you should
reference this variable in a comment at the top of each input file. For
example, an input shell script should begin like this:

#!/bin/sh
# @configure_input@

The presence of that line also reminds people editing the file that it
needs to be processed by configure in order to be used.

— Variable: CPPFLAGS

Preprocessor options for the C, C++, Objective C, and Objective C++
preprocessors and compilers. If
it is not set in the environment when configure runs, the default
value is empty. configure uses this variable when preprocessing
or compiling programs to test for C, C++, Objective C, and Objective C++
features.

This variable's contents should contain options like -I,
-D, and -U that affect only the behavior of the
preprocessor. Please see the explanation of CFLAGS for what you
can do if an option affects other phases of the compiler as well.

Currently, configure always links as part of a single
invocation of the compiler that also preprocesses and compiles, so it
uses this variable also when linking programs. However, it is unwise to
depend on this behavior because the GNU Coding Standards do
not require it and many packages do not use CPPFLAGS when linking
programs.

Debugging and optimization options for the C++ compiler. It acts like
CFLAGS, but for C++ instead of C.

— Variable: DEFS

-D options to pass to the C compiler. If AC_CONFIG_HEADERS
is called, configure replaces ‘@DEFS@’ with
-DHAVE_CONFIG_H instead (see Configuration Headers). This
variable is not defined while configure is performing its tests,
only when creating the output files. See Setting Output Variables, for
how to check the results of previous tests.

— Variable: ECHO_C
— Variable: ECHO_N
— Variable: ECHO_T

How does one suppress the trailing newline from echo for
question-answer message pairs? These variables provide a way:

Some old and uncommon echo implementations offer no means to
achieve this, in which case ECHO_T is set to tab. You might not
want to use it.

— Variable: ERLCFLAGS

Debugging and optimization options for the Erlang compiler. If it is not set
in the environment when configure runs, the default value is empty.
configure uses this variable when compiling
programs to test for Erlang features.

— Variable: FCFLAGS

Debugging and optimization options for the Fortran compiler. If it
is not set in the environment when configure runs, the default
value is set when you call AC_PROG_FC (or empty if you don't).
configure uses this variable when compiling or linking
programs to test for Fortran features.

— Variable: FFLAGS

Debugging and optimization options for the Fortran 77 compiler. If it
is not set in the environment when configure runs, the default
value is set when you call AC_PROG_F77 (or empty if you don't).
configure uses this variable when compiling or linking
programs to test for Fortran 77 features.

— Variable: LDFLAGS

Options for the linker. If it is not set
in the environment when configure runs, the default value is empty.
configure uses this variable when linking programs to test for
C, C++, Objective C, Objective C++, Fortran, and Go features.

This variable's contents should contain options like -s and
-L that affect only the behavior of the linker. Please see the
explanation of CFLAGS for what you can do if an option also
affects other phases of the compiler.

Don't use this variable to pass library names
(-l) to the linker; use LIBS instead.

— Variable: LIBS

-l options to pass to the linker. The default value is empty,
but some Autoconf macros may prepend extra libraries to this variable if
those libraries are found and provide necessary functions, see
Libraries. configure uses this variable when linking
programs to test for C, C++, Objective C, Objective C++, Fortran, and Go
features.

— Variable: OBJCFLAGS

Debugging and optimization options for the Objective C compiler. It
acts like CFLAGS, but for Objective C instead of C.

— Variable: OBJCXXFLAGS

Debugging and optimization options for the Objective C++ compiler. It
acts like CXXFLAGS, but for Objective C++ instead of C++.

— Variable: GOFLAGS

Debugging and optimization options for the Go compiler. It acts like
CFLAGS, but for Go instead of C.

— Variable: builddir

Rigorously equal to ‘.’. Added for symmetry only.

— Variable: abs_builddir

Absolute name of builddir.

— Variable: top_builddir

The relative name of the top level of the current build tree. In the
top-level directory, this is the same as builddir.

— Variable: top_build_prefix

The relative name of the top level of the current build tree with final
slash if nonempty. This is the same as top_builddir, except that
it contains zero or more runs of ../, so it should not be
appended with a slash for concatenation. This helps for make
implementations that otherwise do not treat ./file and file
as equal in the toplevel build directory.

— Variable: abs_top_builddir

Absolute name of top_builddir.

— Variable: srcdir

The name of the directory that contains the source code for
that makefile.

— Variable: abs_srcdir

Absolute name of srcdir.

— Variable: top_srcdir

The name of the top-level source code directory for the
package. In the top-level directory, this is the same as srcdir.

4.8.2 Installation Directory Variables

The following variables specify the directories for
package installation, see Variables for Installation Directories, for more information. Each variable corresponds to an
argument of configure; trailing slashes are stripped so that
expressions such as ‘${prefix}/lib’ expand with only one slash
between directory names. See the end of this section for
details on when and how to use these variables.

The root of the directory tree for read-only architecture-independent
data files.

— Variable: docdir

The directory for installing documentation files (other than Info and
man).

— Variable: dvidir

The directory for installing documentation files in DVI format.

— Variable: exec_prefix

The installation prefix for architecture-dependent files. By default
it's the same as prefix. You should avoid installing anything
directly to exec_prefix. However, the default value for
directories containing architecture-dependent files should be relative
to exec_prefix.

— Variable: htmldir

The directory for installing HTML documentation.

— Variable: includedir

The directory for installing C header files.

— Variable: infodir

The directory for installing documentation in Info format.

— Variable: libdir

The directory for installing object code libraries.

— Variable: libexecdir

The directory for installing executables that other programs run.

— Variable: localedir

The directory for installing locale-dependent but
architecture-independent data, such as message catalogs. This directory
usually has a subdirectory per locale.

— Variable: localstatedir

The directory for installing modifiable single-machine data.

— Variable: mandir

The top-level directory for installing documentation in man format.

— Variable: oldincludedir

The directory for installing C header files for non-GCC compilers.

— Variable: pdfdir

The directory for installing PDF documentation.

— Variable: prefix

The common installation prefix for all files. If exec_prefix
is defined to a different value, prefix is used only for
architecture-independent files.

— Variable: psdir

The directory for installing PostScript documentation.

— Variable: sbindir

The directory for installing executables that system
administrators run.

Most of these variables have values that rely on prefix or
exec_prefix. It is deliberate that the directory output
variables keep them unexpanded: typically ‘@datarootdir@’ is
replaced by ‘${prefix}/share’, not ‘/usr/local/share’, and
‘@datadir@’ is replaced by ‘${datarootdir}’.

This behavior is mandated by the GNU Coding Standards, so that when
the user runs:

‘make’

she can still specify a different prefix from the one specified to
configure, in which case, if needed, the package should hard
code dependencies corresponding to the make-specified prefix.

‘make install’

she can specify a different installation location, in which case the
package must still depend on the location which was compiled in
(i.e., never recompile when ‘make install’ is run). This is an
extremely important feature, as many people may decide to install all
the files of a package grouped together, and then install links from
the final locations to there.

In order to support these features, it is essential that
datarootdir remains defined as ‘${prefix}/share’,
so that its value can be expanded based
on the current value of prefix.

A corollary is that you should not use these variables except in
makefiles. For instance, instead of trying to evaluate datadir
in configure and hard-coding it in makefiles using
e.g., ‘AC_DEFINE_UNQUOTED([DATADIR], ["$datadir"], [Data directory.])’,
you should add
-DDATADIR='$(datadir)' to your makefile's definition of
CPPFLAGS (AM_CPPFLAGS if you are also using Automake).

Similarly, you should not rely on AC_CONFIG_FILES to replace
bindir and friends in your shell scripts and other files; instead,
let make manage their replacement. For instance Autoconf
ships templates of its shell scripts ending with ‘.in’, and uses a
makefile snippet similar to the following to build scripts like
autoheader and autom4te:

4.8.3 Changed Directory Variables

In Autoconf 2.60, the set of directory variables has changed, and the
defaults of some variables have been adjusted
(see Installation Directory Variables) to changes in the
GNU Coding Standards. Notably, datadir, infodir, and
mandir are now expressed in terms of datarootdir. If you are
upgrading from an earlier Autoconf version, you may need to adjust your files
to ensure that the directory variables are substituted correctly
(see Defining Directories), and that a definition of datarootdir is
in place. For example, in a Makefile.in, adding

datarootdir = @datarootdir@

is usually sufficient. If you use Automake to create Makefile.in,
it will add this for you.

To help with the transition, Autoconf warns about files that seem to use
datarootdir without defining it. In some cases, it then expands
the value of $datarootdir in substitutions of the directory
variables. The following example shows such a warning:

In some cases, however, the checks may not be able to detect that a suitable
initialization of datarootdir is in place, or they may fail to detect
that such an initialization is necessary in the output file. If, after
auditing your package, there are still spurious configure warnings about
datarootdir, you may add the line

AC_DEFUN([AC_DATAROOTDIR_CHECKED])

to your configure.ac to disable the warnings. This is an exception
to the usual rule that you should not define a macro whose name begins with
AC_ (see Macro Names).

4.8.4 Build Directories

You can support compiling a software package for several architectures
simultaneously from the same copy of the source code. The object files
for each architecture are kept in their own directory.

To support doing this, make uses the VPATH variable to
find the files that are in the source directory. GNU Make
can do this. Most other recent make programs can do this as
well, though they may have difficulties and it is often simpler to
recommend GNU make (see VPATH and Make). Older
make programs do not support VPATH; when using them, the
source code must be in the same directory as the object files.

If you are using GNU Automake, the remaining details in this
section are already covered for you, based on the contents of your
Makefile.am. But if you are using Autoconf in isolation, then
supporting VPATH requires the following in your
Makefile.in:

configure substitutes the correct value for srcdir when
it produces Makefile.

Do not use the make variable $<, which expands to the
file name of the file in the source directory (found with VPATH),
except in implicit rules. (An implicit rule is one such as ‘.c.o’,
which tells how to create a .o file from a .c file.) Some
versions of make do not set $< in explicit rules; they
expand it to an empty value.

Instead, Make command lines should always refer to source
files by prefixing them with ‘$(srcdir)/’. For example:

4.8.5 Automatic Remaking

You can put rules like the following in the top-level Makefile.in
for a package to automatically update the configuration information when
you change the configuration files. This example includes all of the
optional files, such as aclocal.m4 and those related to
configuration header files. Omit from the Makefile.in rules for
any of these files that your package does not use.

The ‘$(srcdir)/’ prefix is included because of limitations in the
VPATH mechanism.

The stamp- files are necessary because the timestamps of
config.h.in and config.h are not changed if remaking
them does not change their contents. This feature avoids unnecessary
recompilation. You should include the file stamp-h.in in your
package's distribution, so that make considers
config.h.in up to date. Don't use touch
(see Limitations of Usual Tools); instead, use
echo (using
date would cause needless differences, hence CVS
conflicts, etc.).

4.9 Configuration Header Files

When a package contains more than a few tests that define C preprocessor
symbols, the command lines to pass -D options to the compiler
can get quite long. This causes two problems. One is that the
make output is hard to visually scan for errors. More
seriously, the command lines can exceed the length limits of some
operating systems. As an alternative to passing -D options to
the compiler, configure scripts can create a C header file
containing ‘#define’ directives. The AC_CONFIG_HEADERS
macro selects this kind of output. Though it can be called anywhere
between AC_INIT and AC_OUTPUT, it is customary to call
it right after AC_INIT.

The package should ‘#include’ the configuration header file before
any other header files, to prevent inconsistencies in declarations (for
example, if it redefines const).

To provide for VPATH builds, remember to pass the C compiler a -I.
option (or -I..; whichever directory contains config.h).
Even if you use ‘#include "config.h"’, the preprocessor searches only
the directory of the currently read file, i.e., the source directory, not
the build directory.

With the appropriate -I option, you can use
‘#include <config.h>’. Actually, it's a good habit to use it,
because in the rare case when the source directory contains another
config.h, the build directory should be searched first.

— Macro: AC_CONFIG_HEADERS (header ..., [cmds], [init-cmds])

This macro is one of the instantiating macros; see Configuration Actions. Make AC_OUTPUT create the file(s) in the
blank-or-newline-separated list header containing C preprocessor
#define statements, and replace ‘@DEFS@’ in generated
files with -DHAVE_CONFIG_H instead of the value of DEFS.
The usual name for header is config.h.

If header already exists and its contents are identical to what
AC_OUTPUT would put in it, it is left alone. Doing this allows
making some changes in the configuration without needlessly causing
object files that depend on the header file to be recompiled.

Usually the input file is named header.in; however, you can
override the input file name by appending to header a
colon-separated list of input files. For example, you might need to make
the input file name acceptable to DOS variants:

AC_CONFIG_HEADERS([config.h:config.hin])

— Macro: AH_HEADER

This macro is defined as the name of the first declared config header
and undefined if no config headers have been declared up to this point.
A third-party macro may, for example, require use of a config header
without invoking AC_CONFIG_HEADERS twice, like this:

4.9.1 Configuration Header Templates

Your distribution should contain a template file that looks as you want
the final header file to look, including comments, with #undef
statements which are used as hooks. For example, suppose your
configure.ac makes these calls:

AC_CONFIG_HEADERS([conf.h])
AC_CHECK_HEADERS([unistd.h])

Then you could have code like the following in conf.h.in.
The conf.h created by configure defines ‘HAVE_UNISTD_H’
to 1, if and only if the system has unistd.h.

/* Define as 1 if you have unistd.h. */
#undef HAVE_UNISTD_H

The format of the template file is stricter than what the C preprocessor
is required to accept. A directive line should contain only whitespace,
‘#undef’, and ‘HAVE_UNISTD_H’. The use of ‘#define’
instead of ‘#undef’, or of comments on the same line as
‘#undef’, is strongly discouraged. Each hook should only be listed
once. Other preprocessor lines, such as ‘#ifdef’ or
‘#include’, are copied verbatim from the template into the
generated header.

Since it is a tedious task to keep a template header up to date, you may
use autoheader to generate it, see autoheader Invocation.

During the instantiation of the header, each ‘#undef’ line in the
template file for each symbol defined by ‘AC_DEFINE’ is changed to an
appropriate ‘#define’. If the corresponding ‘AC_DEFINE’ has not
been executed during the configure run, the ‘#undef’ line is
commented out. (This is important, e.g., for ‘_POSIX_SOURCE’:
on many systems, it can be implicitly defined by the compiler, and
undefining it in the header would then break compilation of subsequent
headers.)

Currently, all remaining ‘#undef’ lines in the header
template are commented out, whether or not there was a corresponding
‘AC_DEFINE’ for the macro name; but this behavior is not guaranteed
for future releases of Autoconf.

Generally speaking, since you should not use ‘#define’, and you
cannot guarantee whether a ‘#undef’ directive in the header
template will be converted to a ‘#define’ or commented out in the
generated header file, the template file cannot be used for conditional
definition effects. Consequently, if you need to use the construct

#ifdef THIS
# define THAT
#endif

you must place it outside of the template.
If you absolutely need to hook it to the config header itself, please put
the directives to a separate file, and ‘#include’ that file from the
config header template. If you are using autoheader, you would
probably use ‘AH_BOTTOM’ to append the ‘#include’ directive.

4.9.2 Using autoheader to Create config.h.in

The autoheader program can create a template file of C
‘#define’ statements for configure to use.
It searches for the first invocation of AC_CONFIG_HEADERS in
configure sources to determine the name of the template.
(If the first call of AC_CONFIG_HEADERS specifies more than one
input file name, autoheader uses the first one.)

It is recommended that only one input file is used. If you want to append
a boilerplate code, it is preferable to use
‘AH_BOTTOM([#include <conf_post.h>])’.
File conf_post.h is not processed during the configuration then,
which make things clearer. Analogically, AH_TOP can be used to
prepend a boilerplate code.

In order to do its job, autoheader needs you to document all
of the symbols that you might use. Typically this is done via an
AC_DEFINE or AC_DEFINE_UNQUOTED call whose first argument
is a literal symbol and whose third argument describes the symbol
(see Defining Symbols). Alternatively, you can use
AH_TEMPLATE (see Autoheader Macros), or you can supply a
suitable input file for a subsequent configuration header file.
Symbols defined by Autoconf's builtin tests are already documented properly;
you need to document only those that you
define yourself.

You might wonder why autoheader is needed: after all, why
would configure need to “patch” a config.h.in to
produce a config.h instead of just creating config.h from
scratch? Well, when everything rocks, the answer is just that we are
wasting our time maintaining autoheader: generating
config.h directly is all that is needed. When things go wrong,
however, you'll be thankful for the existence of autoheader.

The fact that the symbols are documented is important in order to
check that config.h makes sense. The fact that there is a
well-defined list of symbols that should be defined (or not) is
also important for people who are porting packages to environments where
configure cannot be run: they just have to fill in the
blanks.

But let's come back to the point: the invocation of autoheader...

If you give autoheader an argument, it uses that file instead
of configure.ac and writes the header file to the standard output
instead of to config.h.in. If you give autoheader an
argument of -, it reads the standard input instead of
configure.ac and writes the header file to the standard output.

autoheader accepts the following options:

--help

-h

Print a summary of the command line options and exit.

--version

-V

Print the version number of Autoconf and exit.

--verbose

-v

Report processing steps.

--debug

-d

Don't remove the temporary files.

--force

-f

Remake the template file even if newer than its input files.

--include=dir

-I dir

Append dir to the include path. Multiple invocations accumulate.

--prepend-include=dir

-B dir

Prepend dir to the include path. Multiple invocations accumulate.

--warnings=category

-W category

Report the warnings related to category (which can actually be a
comma separated list). Current categories include:

4.9.3 Autoheader Macros

autoheader scans configure.ac and figures out which C
preprocessor symbols it might define. It knows how to generate
templates for symbols defined by AC_CHECK_HEADERS,
AC_CHECK_FUNCS etc., but if you AC_DEFINE any additional
symbol, you must define a template for it. If there are missing
templates, autoheader fails with an error message.

The template for a symbol is created
by autoheader from
the description argument to an AC_DEFINE;
see Defining Symbols.

For special needs, you can use the following macros.

— Macro: AH_TEMPLATE (key, description)

Tell autoheader to generate a template for key. This macro
generates standard templates just like AC_DEFINE when a
description is given.

For example:

AH_TEMPLATE([CRAY_STACKSEG_END],
[Define to one of _getb67, GETB67, getb67
for Cray-2 and Cray-YMP systems. This
function is required for alloca.c support
on those systems.])

generates the following template, with the description properly
justified.

/* Define to one of _getb67, GETB67, getb67 for Cray-2 and
Cray-YMP systems. This function is required for alloca.c
support on those systems. */
#undef CRAY_STACKSEG_END

— Macro: AH_VERBATIM (key, template)

Tell autoheader to include the template as-is in the header
template file. This template is associated with the key,
which is used to sort all the different templates and guarantee their
uniqueness. It should be a symbol that can be defined via AC_DEFINE.

— Macro: AH_TOP (text)

Include text at the top of the header template file.

— Macro: AH_BOTTOM (text)

Include text at the bottom of the header template file.

Please note that text gets included “verbatim” to the template file,
not to the resulting config header, so it can easily get mangled when the
template is processed. There is rarely a need for something other than

4.10 Running Arbitrary Configuration Commands

You can execute arbitrary commands before, during, and after
config.status is run. The three following macros accumulate the
commands to run when they are called multiple times.
AC_CONFIG_COMMANDS replaces the obsolete macro
AC_OUTPUT_COMMANDS; see Obsolete Macros, for details.

— Macro: AC_CONFIG_COMMANDS (tag..., [cmds], [init-cmds])

Specify additional shell commands to run at the end of
config.status, and shell commands to initialize any variables
from configure. Associate the commands with tag.
Since typically the cmds create a file, tag should
naturally be the name of that file. If needed, the directory hosting
tag is created. This macro is one of the instantiating macros;
see Configuration Actions.

Here is an unrealistic example:

fubar=42
AC_CONFIG_COMMANDS([fubar],
[echo this is extra $fubar, and so on.],
[fubar=$fubar])

Here is a better one:

AC_CONFIG_COMMANDS([timestamp], [date >timestamp])

The following two macros look similar, but in fact they are not of the same
breed: they are executed directly by configure, so you cannot use
config.status to rerun them.

— Macro: AC_CONFIG_COMMANDS_PRE (cmds)

Execute the cmds right before creating config.status.

This macro presents the last opportunity to call AC_SUBST,
AC_DEFINE, or AC_CONFIG_ITEMS macros.

4.11 Creating Configuration Links

You may find it convenient to create links whose destinations depend upon
results of tests. One can use AC_CONFIG_COMMANDS but the
creation of relative symbolic links can be delicate when the package is
built in a directory different from the source directory.

— Macro: AC_CONFIG_LINKS (dest:source..., [cmds], [init-cmds])

Make AC_OUTPUT link each of the existing files source to
the corresponding link name dest. Makes a symbolic link if
possible, otherwise a hard link if possible, otherwise a copy. The
dest and source names should be relative to the top level
source or build directory. This macro is one of the instantiating
macros; see Configuration Actions.

4.12 Configuring Other Packages in Subdirectories

In most situations, calling AC_OUTPUT is sufficient to produce
makefiles in subdirectories. However, configure scripts
that control more than one independent package can use
AC_CONFIG_SUBDIRS to run configure scripts for other
packages in subdirectories.

— Macro: AC_CONFIG_SUBDIRS (dir ...)

Make AC_OUTPUT run configure in each subdirectory
dir in the given blank-or-newline-separated list. Each dir should
be a literal, i.e., please do not use:

If a given dir is not found at configure run time, a
warning is reported; if the subdirectory is optional, write:

if test -d "$srcdir/foo"; then
AC_CONFIG_SUBDIRS([foo])
fi

If a given dir contains configure.gnu, it is run instead
of configure. This is for packages that might use a
non-Autoconf script Configure, which can't be called through a
wrapper configure since it would be the same file on
case-insensitive file systems. Likewise, if a dir contains
configure.in but no configure, the Cygnus
configure script found by AC_CONFIG_AUX_DIR is used.

The subdirectory configure scripts are given the same command
line options that were given to this configure script, with minor
changes if needed, which include:

adjusting a relative name for the cache file;

adjusting a relative name for the source directory;

propagating the current value of $prefix, including if it was
defaulted, and if the default values of the top level and of the subdirectory
configure differ.

This macro also sets the output variable subdirs to the list of
directories ‘dir ...’. Make rules can use
this variable to determine which subdirectories to recurse into.

4.13 Default Prefix

By default, configure sets the prefix for files it installs to
/usr/local. The user of configure can select a different
prefix using the --prefix and --exec-prefix options.
There are two ways to change the default: when creating
configure, and when running it.

Some software packages might want to install in a directory other than
/usr/local by default. To accomplish that, use the
AC_PREFIX_DEFAULT macro.

— Macro: AC_PREFIX_DEFAULT (prefix)

Set the default installation prefix to prefix instead of
/usr/local.

It may be convenient for users to have configure guess the
installation prefix from the location of a related program that they
have already installed. If you wish to do that, you can call
AC_PREFIX_PROGRAM.

— Macro: AC_PREFIX_PROGRAM (program)

If the user did not specify an installation prefix (using the
--prefix option), guess a value for it by looking for
program in PATH, the way the shell does. If program
is found, set the prefix to the parent of the directory containing
program, else default the prefix as described above
(/usr/local or AC_PREFIX_DEFAULT). For example, if
program is gcc and the PATH contains
/usr/local/gnu/bin/gcc, set the prefix to /usr/local/gnu.

5 Existing Tests

These macros test for particular system features that packages might
need or want to use. If you need to test for a kind of feature that
none of these macros check for, you can probably do it by calling
primitive test macros with appropriate arguments (see Writing Tests).

These tests print messages telling the user which feature they're
checking for, and what they find. They cache their results for future
configure runs (see Caching Results).

Some of these macros set output variables. See Makefile Substitutions, for how to get their values. The phrase “define
name” is used below as a shorthand to mean “define the C
preprocessor symbol name to the value 1”. See Defining Symbols, for how to get those symbol definitions into your program.

5.1 Common Behavior

Much effort has been expended to make Autoconf easy to learn. The most
obvious way to reach this goal is simply to enforce standard interfaces
and behaviors, avoiding exceptions as much as possible. Because of
history and inertia, unfortunately, there are still too many exceptions
in Autoconf; nevertheless, this section describes some of the common
rules.

5.1.1 Standard Symbols

All the generic macros that AC_DEFINE a symbol as a result of
their test transform their argument values to a standard alphabet.
First, argument is converted to upper case and any asterisks
(‘*’) are each converted to ‘P’. Any remaining characters
that are not alphanumeric are converted to underscores.

5.2 Alternative Programs

These macros check for the presence or behavior of particular programs.
They are used to choose between several alternative programs and to
decide what to do once one has been chosen. If there is no macro
specifically defined to check for a program you need, and you don't need
to check for any special properties of it, then you can use one of the
general program-check macros.

5.2.1 Particular Program Checks

These macros check for particular programs—whether they exist, and
in some cases whether they support certain features.

— Macro: AC_PROG_AWK

Check for gawk, mawk, nawk, and awk, in that
order, and set output variable AWK to the first one that is found.
It tries gawk first because that is reported to be the
best implementation. The result can be overridden by setting the
variable AWK or the cache variable ac_cv_prog_AWK.

Look for the best available grep or ggrep that accepts the
longest input lines possible, and that supports multiple -e options.
Set the output variable GREP to whatever is chosen.
See Limitations of Usual Tools, for more information about
portability problems with the grep command family. The result
can be overridden by setting the GREP variable and is cached in the
ac_cv_path_GREP variable.

— Macro: AC_PROG_EGREP

Check whether $GREP -E works, or else look for the best available
egrep or gegrep that accepts the longest input lines possible.
Set the output variable EGREP to whatever is chosen. The result
can be overridden by setting the EGREP variable and is cached in the
ac_cv_path_EGREP variable.

— Macro: AC_PROG_FGREP

Check whether $GREP -F works, or else look for the best available
fgrep or gfgrep that accepts the longest input lines possible.
Set the output variable FGREP to whatever is chosen. The result
can be overridden by setting the FGREP variable and is cached in the
ac_cv_path_FGREP variable.

— Macro: AC_PROG_INSTALL

Set output variable INSTALL to the name of a BSD-compatible
install program, if one is found in the current PATH.
Otherwise, set INSTALL to ‘dir/install-sh -c’,
checking the directories specified to AC_CONFIG_AUX_DIR (or its
default directories) to determine dir (see Output). Also set
the variables INSTALL_PROGRAM and INSTALL_SCRIPT to
‘${INSTALL}’ and INSTALL_DATA to ‘${INSTALL} -m 644’.

‘@INSTALL@’ is special, as its value may vary for different
configuration files.

This macro screens out various instances of install known not to
work. It prefers to find a C program rather than a shell script, for
speed. Instead of install-sh, it can also use install.sh,
but that name is obsolete because some make programs have a rule
that creates install from it if there is no makefile. Further, this
macro requires install to be able to install multiple files into a
target directory in a single invocation.

Autoconf comes with a copy of install-sh that you can use. If
you use AC_PROG_INSTALL, you must include either
install-sh or install.sh in your distribution; otherwise
configure produces an error message saying it can't find
them—even if the system you're on has a good install program.
This check is a safety measure to prevent you from accidentally leaving
that file out, which would prevent your package from installing on
systems that don't have a BSD-compatible install program.

If you need to use your own installation program because it has features
not found in standard install programs, there is no reason to use
AC_PROG_INSTALL; just put the file name of your program into your
Makefile.in files.

The result of the test can be overridden by setting the variable
INSTALL or the cache variable ac_cv_path_install.

— Macro: AC_PROG_MKDIR_P

Set output variable MKDIR_P to a program that ensures that for
each argument, a directory named by this argument exists, creating it
and its parent directories if needed, and without race conditions when
two instances of the program attempt to make the same directory at
nearly the same time.

This macro uses the ‘mkdir -p’ command if possible. Otherwise, it
falls back on invoking install-sh with the -d option,
so your package should
contain install-sh as described under AC_PROG_INSTALL.
An install-sh file that predates Autoconf 2.60 or Automake 1.10
is vulnerable to race conditions, so if you want to support parallel
installs from
different packages into the same directory you need to make sure you
have an up-to-date install-sh. In particular, be careful about
using ‘autoreconf -if’ if your Automake predates Automake 1.10.

This macro is related to the AS_MKDIR_P macro (see Programming in M4sh), but it sets an output variable intended for use in other
files, whereas AS_MKDIR_P is intended for use in scripts like
configure. Also, AS_MKDIR_P does not accept options,
but MKDIR_P supports the -m option, e.g., a makefile
might invoke $(MKDIR_P) -m 0 dir to create an inaccessible
directory, and conversely a makefile should use $(MKDIR_P) --
$(FOO) if FOO might yield a value that begins with ‘-’.
Finally, AS_MKDIR_P does not check for race condition
vulnerability, whereas AC_PROG_MKDIR_P does.

‘@MKDIR_P@’ is special, as its value may vary for different
configuration files.

The result of the test can be overridden by setting the variable
MKDIR_P or the cache variable ac_cv_path_mkdir.

— Macro: AC_PROG_LEX

If flex is found, set output variable LEX to ‘flex’
and LEXLIB to -lfl, if that library is in a standard
place. Otherwise set LEX to ‘lex’ and LEXLIB to
-ll, if found. If neither variant is available, set LEX
to ‘:’; for packages that ship the generated file.yy.c
alongside the source file.l, this default allows users without a
lexer generator to still build the package even if the timestamp for
file.l is inadvertently changed.

Define YYTEXT_POINTER if yytext defaults to ‘char *’ instead
of to ‘char []’. Also set output variable LEX_OUTPUT_ROOT to
the base of the file name that the lexer generates; usually
lex.yy, but sometimes something else. These results vary
according to whether lex or flex is being used.

You are encouraged to use Flex in your sources, since it is both more
pleasant to use than plain Lex and the C source it produces is portable.
In order to ensure portability, however, you must either provide a
function yywrap or, if you don't use it (e.g., your scanner has
no ‘#include’-like feature), simply include a ‘%noyywrap’
statement in the scanner's source. Once this done, the scanner is
portable (unless you felt free to use nonportable constructs) and
does not depend on any library. In this case, and in this case only, it
is suggested that you use this Autoconf snippet:

Remember that the user may have supplied an alternate location in
LEX, so if Flex is required, it is better to check that the user
provided something sufficient by parsing the output of ‘$LEX
--version’ than by simply relying on test "x$LEX" = xflex.

As part of running the test, this macro may delete any file in the
configuration directory named lex.yy.c or lexyy.c.

The result of this test can be influenced by setting the variable
LEX or the cache variable ac_cv_prog_LEX.

— Macro: AC_PROG_LN_S

If ‘ln -s’ works on the current file system (the operating system
and file system support symbolic links), set the output variable
LN_S to ‘ln -s’; otherwise, if ‘ln’ works, set
LN_S to ‘ln’, and otherwise set it to ‘cp -pR’.

If you make a link in a directory other than the current directory, its
meaning depends on whether ‘ln’ or ‘ln -s’ is used. To safely
create links using ‘$(LN_S)’, either find out which form is used
and adjust the arguments, or always invoke ln in the directory
where the link is to be created.

In other words, it does not work to do:

$(LN_S) foo /x/bar

Instead, do:

(cd /x && $(LN_S) foo bar)

— Macro: AC_PROG_RANLIB

Set output variable RANLIB to ‘ranlib’ if ranlib
is found, and otherwise to ‘:’ (do nothing).

— Macro: AC_PROG_SED

Set output variable SED to a Sed implementation that conforms to
Posix and does not have arbitrary length limits. Report an error if no
acceptable Sed is found. See Limitations of Usual Tools, for more
information about portability problems with Sed.

The result of this test can be overridden by setting the SED variable
and is cached in the ac_cv_path_SED variable.

— Macro: AC_PROG_YACC

If bison is found, set output variable YACC to ‘bison
-y’. Otherwise, if byacc is found, set YACC to
‘byacc’. Otherwise set YACC to ‘yacc’.
The result of this test can be influenced by setting the variable
YACC or the cache variable ac_cv_prog_YACC.

5.2.2 Generic Program and File Checks

These macros are used to find programs not covered by the “particular”
test macros. If you need to check the behavior of a program as well as
find out whether it is present, you have to write your own test for it
(see Writing Tests). By default, these macros use the environment
variable PATH. If you need to check for a program that might not
be in the user's PATH, you can pass a modified path to use
instead, like this:

Check whether program prog-to-check-for exists in path. If
it is found, set variable to value-if-found, otherwise to
value-if-not-found, if given. Always pass over reject (an
absolute file name) even if it is the first found in the search path; in
that case, set variable using the absolute file name of the
prog-to-check-for found that is not reject. If
variable was already set, do nothing. Calls AC_SUBST for
variable. The result of this test can be overridden by setting the
variable variable or the cache variable
ac_cv_prog_variable.

Check for each program in the blank-separated list
progs-to-check-for existing in the path. If one is found, set
variable to the name of that program. Otherwise, continue
checking the next program in the list. If none of the programs in the
list are found, set variable to value-if-not-found; if
value-if-not-found is not specified, the value of variable
is not changed. Calls AC_SUBST for variable. The result of
this test can be overridden by setting the variable variable or the
cache variable ac_cv_prog_variable.

Like AC_CHECK_PROG, but first looks for prog-to-check-for
with a prefix of the target type as determined by
AC_CANONICAL_TARGET, followed by a dash (see Canonicalizing).
If the tool cannot be found with a prefix, and if the build and target
types are equal, then it is also searched for without a prefix.

As noted in Specifying Target Triplets, the
target is rarely specified, because most of the time it is the same
as the host: it is the type of system for which any compiler tool in
the package produces code. What this macro looks for is,
for example, a tool (assembler, linker, etc.) that the
compiler driver (gcc for the GNU C Compiler)
uses to produce objects, archives or executables.

Like AC_CHECK_PROG, but first looks for prog-to-check-for
with a prefix of the host type as specified by --host, followed by a
dash. For example, if the user runs
‘configure --build=x86_64-gnu --host=i386-gnu’, then this call:

AC_CHECK_TOOL([RANLIB], [ranlib], [:])

sets RANLIB to i386-gnu-ranlib if that program exists in
path, or otherwise to ‘ranlib’ if that program exists in
path, or to ‘:’ if neither program exists.

When cross-compiling, this macro will issue a warning if no program
prefixed with the host type could be found.
For more information, see Specifying Target Triplets.

Like AC_CHECK_TARGET_TOOL, each of the tools in the list
progs-to-check-for are checked with a prefix of the target type as
determined by AC_CANONICAL_TARGET, followed by a dash
(see Canonicalizing). If none of the tools can be found with a
prefix, and if the build and target types are equal, then the first one
without a prefix is used. If a tool is found, set variable to
the name of that program. If none of the tools in the list are found,
set variable to value-if-not-found; if value-if-not-found
is not specified, the value of variable is not changed. Calls
AC_SUBST for variable.

Like AC_CHECK_TOOL, each of the tools in the list
progs-to-check-for are checked with a prefix of the host type as
determined by AC_CANONICAL_HOST, followed by a dash
(see Canonicalizing). If none of the tools can be found with a
prefix, then the first one without a prefix is used. If a tool is found,
set variable to the name of that program. If none of the tools in
the list are found, set variable to value-if-not-found; if
value-if-not-found is not specified, the value of variable
is not changed. Calls AC_SUBST for variable.

When cross-compiling, this macro will issue a warning if no program
prefixed with the host type could be found.
For more information, see Specifying Target Triplets.

Like AC_CHECK_PROG, but set variable to the absolute
name of prog-to-check-for if found. The result of this test
can be overridden by setting the variable variable. A positive
result of this test is cached in the ac_cv_path_variable
variable.

Like AC_CHECK_PROGS, but if any of progs-to-check-for
are found, set variable to the absolute name of the program
found. The result of this test can be overridden by setting the
variable variable. A positive result of this test is cached in
the ac_cv_path_variable variable.

This macro was introduced in Autoconf 2.62. If variable is not
empty, then set the cache variable ac_cv_path_variable to
its value. Otherwise, check for each program in the blank-separated
list progs-to-check-for existing in path. For each program
found, execute feature-test with ac_path_variable
set to the absolute name of the candidate program. If no invocation of
feature-test sets the shell variable
ac_cv_path_variable, then action-if-not-found is
executed. feature-test will be run even when
ac_cv_path_variable is set, to provide the ability to
choose a better candidate found later in path; to accept the
current setting and bypass all further checks, feature-test can
execute ac_path_variable_found=:.

Note that this macro has some subtle differences from
AC_CHECK_PROGS. It is designed to be run inside
AC_CACHE_VAL, therefore, it should have no side effects. In
particular, variable is not set to the final value of
ac_cv_path_variable, nor is AC_SUBST automatically
run. Also, on failure, any action can be performed, whereas
AC_CHECK_PROGS only performs
variable=value-if-not-found.

Here is an example, similar to what Autoconf uses in its own configure
script. It will search for an implementation of m4 that
supports the indir builtin, even if it goes by the name
gm4 or is not the first implementation on PATH.

5.3 Files

You might also need to check for the existence of files. Before using
these macros, ask yourself whether a runtime test might not be a better
solution. Be aware that, like most Autoconf macros, they test a feature
of the host machine, and therefore, they die when cross-compiling.

Check whether file file exists on the native system. If it is
found, execute action-if-found, otherwise do
action-if-not-found, if given. The result of this test is cached
in the ac_cv_file_file variable, with characters not
suitable for a variable name mapped to underscores.

Executes AC_CHECK_FILE once for each file listed in files.
Additionally, defines ‘HAVE_file’ (see Standard Symbols)
for each file found. The results of each test are cached in the
ac_cv_file_file variable, with characters not suitable for
a variable name mapped to underscores.

Test whether the library library is available by trying to link
a test program that calls function function with the library.
function should be a function provided by the library.
Use the base
name of the library; e.g., to check for -lmp, use ‘mp’ as
the library argument.

action-if-found is a list of shell commands to run if the link
with the library succeeds; action-if-not-found is a list of shell
commands to run if the link fails. If action-if-found is not
specified, the default action prepends -llibrary to
LIBS and defines ‘HAVE_LIBlibrary’ (in all
capitals). This macro is intended to support building LIBS in
a right-to-left (least-dependent to most-dependent) fashion such that
library dependencies are satisfied as a natural side effect of
consecutive tests. Linkers are sensitive to library ordering
so the order in which LIBS is generated is important to reliable
detection of libraries.

If linking with library results in unresolved symbols that would
be resolved by linking with additional libraries, give those libraries
as the other-libraries argument, separated by spaces:
e.g., -lXt -lX11. Otherwise, this macro may fail to detect
that library is present, because linking the test program can
fail with unresolved symbols. The other-libraries argument
should be limited to cases where it is desirable to test for one library
in the presence of another that is not already in LIBS.

AC_CHECK_LIB requires some care in usage, and should be avoided
in some common cases. Many standard functions like gethostbyname
appear in the standard C library on some hosts, and in special libraries
like nsl on other hosts. On some hosts the special libraries
contain variant implementations that you may not want to use. These
days it is normally better to use AC_SEARCH_LIBS([gethostbyname],
[nsl]) instead of AC_CHECK_LIB([nsl], [gethostbyname]).

The result of this test is cached in the
ac_cv_lib_library_function variable.

Search for a library defining function if it's not already
available. This equates to calling
‘AC_LINK_IFELSE([AC_LANG_CALL([], [function])])’ first with
no libraries, then for each library listed in search-libs.

Prepend -llibrary to LIBS for the first library found
to contain function, and run action-if-found. If the
function is not found, run action-if-not-found.

If linking with library results in unresolved symbols that would
be resolved by linking with additional libraries, give those libraries
as the other-libraries argument, separated by spaces:
e.g., -lXt -lX11. Otherwise, this macro fails to detect
that function is present, because linking the test program
always fails with unresolved symbols.

The result of this test is cached in the
ac_cv_search_function variable as ‘none required’ if
function is already available, as ‘no’ if no library
containing function was found, otherwise as the
-llibrary option that needs to be prepended to LIBS.

5.5 Library Functions

The following macros check for particular C library functions.
If there is no macro specifically defined to check for a function you need,
and you don't need to check for any special properties of
it, then you can use one of the general function-check macros.

5.5.1 Portability of C Functions

Most usual functions can either be missing, or be buggy, or be limited
on some architectures. This section tries to make an inventory of these
portability issues. By definition, this list always requires
additions. A much more complete list is maintained by the Gnulib
project (see Gnulib), covering Current Posix Functions, Legacy Functions, and Glibc Functions. Please
help us keep the gnulib list as complete as possible.

exit

On ancient hosts, exit returned int.
This is because exit predates void, and there was a long
tradition of it returning int.

On current hosts, the problem more likely is that exit is not
declared, due to C++ problems of some sort or another. For this reason
we suggest that test programs not invoke exit, but return from
main instead.

free

The C standard says a call free (NULL) does nothing, but
some old systems don't support this (e.g., NextStep).

isinf

isnan

The C99 standard says that isinf and isnan are
macros. On some systems just macros are available
(e.g., HP-UX and Solaris 10), on
some systems both macros and functions (e.g., glibc 2.3.2), and on some
systems only functions (e.g., IRIX 6 and Solaris 9). In some cases
these functions are declared in nonstandard headers like
<sunmath.h> and defined in non-default libraries like
-lm or -lsunmath.

The C99 isinf and isnan macros work correctly with
long double arguments, but pre-C99 systems that use functions
typically assume double arguments. On such a system,
isinf incorrectly returns true for a finite long double
argument that is outside the range of double.

The best workaround for these issues is to use gnulib modules
isinf and isnan (see Gnulib). But a lighter weight
solution involves code like the following.

Use AC_C_INLINE (see C Compiler) so that this code works on
compilers that lack the inline keyword. Some optimizing
compilers mishandle these definitions, but systems with that bug
typically have many other floating point corner-case compliance problems
anyway, so it's probably not worth worrying about.

malloc

The C standard says a call malloc (0) is implementation
dependent. It can return either NULL or a new non-null pointer.
The latter is more common (e.g., the GNU C Library) but is by
no means universal. AC_FUNC_MALLOC
can be used to insist on non-NULL (see Particular Functions).

putenv

Posix prefers setenv to putenv; among other things,
putenv is not required of all Posix implementations, but
setenv is.

Posix specifies that putenv puts the given string directly in
environ, but some systems make a copy of it instead (e.g.,
glibc 2.0, or BSD). And when a copy is made, unsetenv might
not free it, causing a memory leak (e.g., FreeBSD 4).

On some systems putenv ("FOO") removes ‘FOO’ from the
environment, but this is not standard usage and it dumps core
on some systems (e.g., AIX).

On MinGW, a call putenv ("FOO=") removes ‘FOO’ from the
environment, rather than inserting it with an empty value.

realloc

The C standard says a call realloc (NULL, size) is equivalent
to malloc (size), but some old systems don't support this (e.g.,
NextStep).

signal handler

Normally signal takes a handler function with a return type of
void, but some old systems required int instead. Any
actual int value returned is not used; this is only a
difference in the function prototype demanded.

All systems we know of in current use return void. The
int was to support K&R C, where of course void is not
available. The obsolete macro AC_TYPE_SIGNAL
(see AC_TYPE_SIGNAL) can be used to establish the correct type in
all cases.

In most cases, it is more robust to use sigaction when it is
available, rather than signal.

snprintf

The C99 standard says that if the output array isn't big enough
and if no other errors occur, snprintf and vsnprintf
truncate the output and return the number of bytes that ought to have
been produced. Some older systems return the truncated length (e.g.,
GNU C Library 2.0.x or IRIX 6.5), some a negative value
(e.g., earlier GNU C Library versions), and some the buffer
length without truncation (e.g., 32-bit Solaris 7). Also, some buggy
older systems ignore the length and overrun the buffer (e.g., 64-bit
Solaris 7).

sprintf

The C standard says sprintf and vsprintf return the
number of bytes written. On some ancient systems (SunOS 4 for
instance) they return the buffer pointer instead, but these no
longer need to be worried about.

sscanf

On various old systems, e.g., HP-UX 9, sscanf requires
that its
input string be writable (though it doesn't actually change it). This
can be a problem when using gcc since it normally puts
constant strings in read-only memory (see Incompatibilities of GCC). Apparently in some cases even
having format strings read-only can be a problem.

strerror_r

Posix specifies that strerror_r returns an int, but many
systems (e.g., GNU C Library version 2.2.4) provide a
different version returning a char *. AC_FUNC_STRERROR_R
can detect which is in use (see Particular Functions).

strnlen

AIX 4.3 provides a broken version which produces the
following results:

_SC_PAGESIZE is standard, but some older systems (e.g., HP-UX
9) have _SC_PAGE_SIZE instead. This can be tested with
#ifdef.

unlink

The Posix spec says that unlink causes the given file to be
removed only after there are no more open file handles for it. Some
non-Posix hosts have trouble with this requirement, though,
and some DOS variants even corrupt the file system.

unsetenv

On MinGW, unsetenv is not available, but a variable ‘FOO’
can be removed with a call putenv ("FOO="), as described under
putenv above.

va_copy

The C99 standard provides va_copy for copying
va_list variables. It may be available in older environments
too, though possibly as __va_copy (e.g., gcc in strict
pre-C99 mode). These can be tested with #ifdef. A fallback to
memcpy (&dst, &src, sizeof (va_list)) gives maximum
portability.

va_list

va_list is not necessarily just a pointer. It can be a
struct (e.g., gcc on Alpha), which means NULL is
not portable. Or it can be an array (e.g., gcc in some
PowerPC configurations), which means as a function parameter it can be
effectively call-by-reference and library routines might modify the
value back in the caller (e.g., vsnprintf in the GNU C Library
2.1).

Signed >>

Normally the C >> right shift of a signed type replicates the
high bit, giving a so-called “arithmetic” shift. But care should be
taken since Standard C doesn't require that behavior. On those
few processors without a native arithmetic shift (for instance Cray
vector systems) zero bits may be shifted in, the same as a shift of an
unsigned type.

Integer /

C divides signed integers by truncating their quotient toward zero,
yielding the same result as Fortran. However, before C99 the standard
allowed C implementations to take the floor or ceiling of the quotient
in some cases. Hardly any implementations took advantage of this
freedom, though, and it's probably not worth worrying about this issue
nowadays.

5.5.2 Particular Function Checks

These macros check for particular C functions—whether they exist, and
in some cases how they respond when given certain arguments.

— Macro: AC_FUNC_ALLOCA

Check how to get alloca. Tries to get a builtin version by
checking for alloca.h or the predefined C preprocessor macros
__GNUC__ and _AIX. If this macro finds alloca.h,
it defines HAVE_ALLOCA_H.

If those attempts fail, it looks for the function in the standard C
library. If any of those methods succeed, it defines
HAVE_ALLOCA. Otherwise, it sets the output variable
ALLOCA to ‘${LIBOBJDIR}alloca.o’ and defines
C_ALLOCA (so programs can periodically call ‘alloca (0)’ to
garbage collect). This variable is separate from LIBOBJS so
multiple programs can share the value of ALLOCA without needing
to create an actual library, in case only some of them use the code in
LIBOBJS. The ‘${LIBOBJDIR}’ prefix serves the same
purpose as in LIBOBJS (see AC_LIBOBJ vs LIBOBJS).

This macro does not try to get alloca from the System V R3
libPW or the System V R4 libucb because those libraries
contain some incompatible functions that cause trouble. Some versions
do not even contain alloca or contain a buggy version. If you
still want to use their alloca, use ar to extract
alloca.o from them instead of compiling alloca.c.

Source files that use alloca should start with a piece of code
like the following, to declare it properly.

If the chown function is available and works (in particular, it
should accept -1 for uid and gid), define
HAVE_CHOWN. The result of this macro is cached in the
ac_cv_func_chown_works variable.

— Macro: AC_FUNC_CLOSEDIR_VOID

If the closedir function does not return a meaningful value,
define CLOSEDIR_VOID. Otherwise, callers ought to check its
return value for an error indicator.

Currently this test is implemented by running a test program. When
cross compiling the pessimistic assumption that closedir does not
return a meaningful value is made.

The result of this macro is cached in the ac_cv_func_closedir_void
variable.

This macro is obsolescent, as closedir returns a meaningful value
on current systems. New programs need not use this macro.

— Macro: AC_FUNC_ERROR_AT_LINE

If the error_at_line function is not found, require an
AC_LIBOBJ replacement of ‘error’.

The result of this macro is cached in the ac_cv_lib_error_at_line
variable.

The AC_FUNC_ERROR_AT_LINE macro is obsolescent. New programs
should use Gnulib's error module. See Gnulib.

— Macro: AC_FUNC_FNMATCH

If the fnmatch function conforms to Posix, define
HAVE_FNMATCH. Detect common implementation bugs, for example,
the bugs in Solaris 2.4.

Unlike the other specific
AC_FUNC macros, AC_FUNC_FNMATCH does not replace a
broken/missing fnmatch. This is for historical reasons.
See AC_REPLACE_FNMATCH below.

The result of this macro is cached in the ac_cv_func_fnmatch_works
variable.

This macro is obsolescent. New programs should use Gnulib's
fnmatch-posix module. See Gnulib.

— Macro: AC_FUNC_FNMATCH_GNU

Behave like AC_REPLACE_FNMATCH (replace) but also test
whether fnmatch supports GNU extensions. Detect common
implementation bugs, for example, the bugs in the GNU C
Library 2.1.

The result of this macro is cached in the ac_cv_func_fnmatch_gnu
variable.

This macro is obsolescent. New programs should use Gnulib's
fnmatch-gnu module. See Gnulib.

— Macro: AC_FUNC_FORK

This macro checks for the fork and vfork functions. If a
working fork is found, define HAVE_WORKING_FORK. This macro
checks whether fork is just a stub by trying to run it.

If vfork.h is found, define HAVE_VFORK_H. If a working
vfork is found, define HAVE_WORKING_VFORK. Otherwise,
define vfork to be fork for backward compatibility with
previous versions of autoconf. This macro checks for several known
errors in implementations of vfork and considers the system to not
have a working vfork if it detects any of them. It is not considered
to be an implementation error if a child's invocation of signal
modifies the parent's signal handler, since child processes rarely change
their signal handlers.

Since this macro defines vfork only for backward compatibility with
previous versions of autoconf you're encouraged to define it
yourself in new code:

#ifndef HAVE_WORKING_VFORK
# define vfork fork
#endif

The results of this macro are cached in the ac_cv_func_fork_works
and ac_cv_func_vfork_works variables. In order to override the
test, you also need to set the ac_cv_func_fork and
ac_cv_func_vfork variables.

— Macro: AC_FUNC_FSEEKO

If the fseeko function is available, define HAVE_FSEEKO.
Define _LARGEFILE_SOURCE if necessary to make the prototype
visible on some systems (e.g., glibc 2.2). Otherwise linkage problems
may occur when compiling with AC_SYS_LARGEFILE on
largefile-sensitive systems where off_t does not default to a
64bit entity. All systems with fseeko also supply ftello.

— Macro: AC_FUNC_GETGROUPS

If the getgroups function is available and works (unlike on
Ultrix 4.3, where ‘getgroups (0, 0)’ always fails), define
HAVE_GETGROUPS. Set GETGROUPS_LIBS to any libraries
needed to get that function. This macro runs AC_TYPE_GETGROUPS.

— Macro: AC_FUNC_GETLOADAVG

Check how to get the system load averages. To perform its tests
properly, this macro needs the file getloadavg.c; therefore, be
sure to set the AC_LIBOBJ replacement directory properly (see
Generic Functions, AC_CONFIG_LIBOBJ_DIR).

If the system has the getloadavg function, define
HAVE_GETLOADAVG, and set GETLOADAVG_LIBS to any libraries
necessary to get that function. Also add GETLOADAVG_LIBS to
LIBS. Otherwise, require an AC_LIBOBJ replacement for
‘getloadavg’ with source code in dir/getloadavg.c, and
possibly define several other C preprocessor macros and output
variables:

Define C_GETLOADAVG.

Define SVR4, DGUX, UMAX, or UMAX4_3 if on
those systems.

If nlist.h is found, define HAVE_NLIST_H.

If ‘struct nlist’ has an ‘n_un.n_name’ member, define
HAVE_STRUCT_NLIST_N_UN_N_NAME. The obsolete symbol
NLIST_NAME_UNION is still defined, but do not depend upon it.

Programs may need to be installed set-group-ID (or set-user-ID) for
getloadavg to work. In this case, define
GETLOADAVG_PRIVILEGED, set the output variable NEED_SETGID
to ‘true’ (and otherwise to ‘false’), and set
KMEM_GROUP to the name of the group that should own the installed
program.

The AC_FUNC_GETLOADAVG macro is obsolescent. New programs should
use Gnulib's getloadavg module. See Gnulib.

— Macro: AC_FUNC_GETMNTENT

Check for getmntent in the standard C library, and then in the
sun, seq, and gen libraries, for UNICOS,
IRIX 4, PTX, and UnixWare, respectively. Then, if
getmntent is available, define HAVE_GETMNTENT and set
ac_cv_func_getmntent to yes. Otherwise set
ac_cv_func_getmntent to no.

The result of this macro can be overridden by setting the cache variable
ac_cv_search_getmntent.

— Macro: AC_FUNC_GETPGRP

Define GETPGRP_VOID if it is an error to pass 0 to
getpgrp; this is the Posix behavior. On older BSD
systems, you must pass 0 to getpgrp, as it takes an argument and
behaves like Posix's getpgid.

#ifdef GETPGRP_VOID
pid = getpgrp ();
#else
pid = getpgrp (0);
#endif

This macro does not check whether
getpgrp exists at all; if you need to work in that situation,
first call AC_CHECK_FUNC for getpgrp.

The result of this macro is cached in the ac_cv_func_getpgrp_void
variable.

This macro is obsolescent, as current systems have a getpgrp
whose signature conforms to Posix. New programs need not use this macro.

— Macro: AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK

If link is a symbolic link, then lstat should treat
link/ the same as link/.. However, many older
lstat implementations incorrectly ignore trailing slashes.

It is safe to assume that if lstat incorrectly ignores
trailing slashes, then other symbolic-link-aware functions like
unlink also incorrectly ignore trailing slashes.

The result of this macro is cached in the
ac_cv_func_lstat_dereferences_slashed_symlink variable.

The AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK macro is obsolescent.
New programs should use Gnulib's lstat module. See Gnulib.

— Macro: AC_FUNC_MALLOC

If the malloc function is compatible with the GNU C
library malloc (i.e., ‘malloc (0)’ returns a valid
pointer), define HAVE_MALLOC to 1. Otherwise define
HAVE_MALLOC to 0, ask for an AC_LIBOBJ replacement for
‘malloc’, and define malloc to rpl_malloc so that the
native malloc is not used in the main project.

Typically, the replacement file malloc.c should look like (note
the ‘#undef malloc’):

The result of this macro is cached in the
ac_cv_func_malloc_0_nonnull variable.

— Macro: AC_FUNC_MBRTOWC

Define HAVE_MBRTOWC to 1 if the function mbrtowc and the
type mbstate_t are properly declared.

The result of this macro is cached in the ac_cv_func_mbrtowc
variable.

— Macro: AC_FUNC_MEMCMP

If the memcmp function is not available, or does not work on
8-bit data (like the one on SunOS 4.1.3), or fails when comparing 16
bytes or more and with at least one buffer not starting on a 4-byte
boundary (such as the one on NeXT x86 OpenStep), require an
AC_LIBOBJ replacement for ‘memcmp’.

The result of this macro is cached in the
ac_cv_func_memcmp_working variable.

This macro is obsolescent, as current systems have a working
memcmp. New programs need not use this macro.

— Macro: AC_FUNC_MKTIME

If the mktime function is not available, or does not work
correctly, require an AC_LIBOBJ replacement for ‘mktime’.
For the purposes of this test, mktime should conform to the
Posix standard and should be the inverse of
localtime.

The result of this macro is cached in the
ac_cv_func_working_mktime variable.

The AC_FUNC_MKTIME macro is obsolescent. New programs should
use Gnulib's mktime module. See Gnulib.

— Macro: AC_FUNC_MMAP

If the mmap function exists and works correctly, define
HAVE_MMAP. This checks only private fixed mapping of already-mapped
memory.

The result of this macro is cached in the
ac_cv_func_mmap_fixed_mapped variable.

— Macro: AC_FUNC_OBSTACK

If the obstacks are found, define HAVE_OBSTACK, else require an
AC_LIBOBJ replacement for ‘obstack’.

The result of this macro is cached in the ac_cv_func_obstack
variable.

— Macro: AC_FUNC_REALLOC

If the realloc function is compatible with the GNU C
library realloc (i.e., ‘realloc (NULL, 0)’ returns a
valid pointer), define HAVE_REALLOC to 1. Otherwise define
HAVE_REALLOC to 0, ask for an AC_LIBOBJ replacement for
‘realloc’, and define realloc to rpl_realloc so that
the native realloc is not used in the main project. See
AC_FUNC_MALLOC for details.

The result of this macro is cached in the
ac_cv_func_realloc_0_nonnull variable.

— Macro: AC_FUNC_SELECT_ARGTYPES

Determines the correct type to be passed for each of the
select function's arguments, and defines those types
in SELECT_TYPE_ARG1, SELECT_TYPE_ARG234, and
SELECT_TYPE_ARG5 respectively. SELECT_TYPE_ARG1 defaults
to ‘int’, SELECT_TYPE_ARG234 defaults to ‘int *’,
and SELECT_TYPE_ARG5 defaults to ‘struct timeval *’.

This macro is obsolescent, as current systems have a select whose
signature conforms to Posix. New programs need not use this macro.

— Macro: AC_FUNC_SETPGRP

If setpgrp takes no argument (the Posix version), define
SETPGRP_VOID. Otherwise, it is the BSD version, which takes
two process IDs as arguments. This macro does not check whether
setpgrp exists at all; if you need to work in that situation,
first call AC_CHECK_FUNC for setpgrp.

The result of this macro is cached in the ac_cv_func_setpgrp_void
variable.

This macro is obsolescent, as current systems have a setpgrp
whose signature conforms to Posix. New programs need not use this macro.

— Macro: AC_FUNC_STAT
— Macro: AC_FUNC_LSTAT

Determine whether stat or lstat have the bug that it
succeeds when given the zero-length file name as argument. The stat
and lstat from SunOS 4.1.4 and the Hurd (as of 1998-11-01) do
this.

If it does, then define HAVE_STAT_EMPTY_STRING_BUG (or
HAVE_LSTAT_EMPTY_STRING_BUG) and ask for an AC_LIBOBJ
replacement of it.

The results of these macros are cached in the
ac_cv_func_stat_empty_string_bug and the
ac_cv_func_lstat_empty_string_bug variables, respectively.

These macros are obsolescent, as no current systems have the bug.
New programs need not use these macros.

— Macro: AC_FUNC_STRCOLL

If the strcoll function exists and works correctly, define
HAVE_STRCOLL. This does a bit more than
‘AC_CHECK_FUNCS(strcoll)’, because some systems have incorrect
definitions of strcoll that should not be used.

The result of this macro is cached in the ac_cv_func_strcoll_works
variable.

— Macro: AC_FUNC_STRERROR_R

If strerror_r is available, define HAVE_STRERROR_R, and if
it is declared, define HAVE_DECL_STRERROR_R. If it returns a
char * message, define STRERROR_R_CHAR_P; otherwise it
returns an int error number. The Thread-Safe Functions option of
Posix requires strerror_r to return int, but
many systems (including, for example, version 2.2.4 of the GNU C
Library) return a char * value that is not necessarily equal to
the buffer argument.

The result of this macro is cached in the
ac_cv_func_strerror_r_char_p variable.

— Macro: AC_FUNC_STRFTIME

Check for strftime in the intl library, for SCO Unix.
Then, if strftime is available, define HAVE_STRFTIME.

This macro is obsolescent, as no current systems require the intl
library for strftime. New programs need not use this macro.

— Macro: AC_FUNC_STRTOD

If the strtod function does not exist or doesn't work correctly,
ask for an AC_LIBOBJ replacement of ‘strtod’. In this case,
because strtod.c is likely to need ‘pow’, set the output
variable POW_LIB to the extra library needed.

This macro caches its result in the ac_cv_func_strtod variable
and depends upon the result in the ac_cv_func_pow variable.

The AC_FUNC_STRTOD macro is obsolescent. New programs should
use Gnulib's strtod module. See Gnulib.

— Macro: AC_FUNC_STRTOLD

If the strtold function exists and conforms to C99, define
HAVE_STRTOLD.

This macro caches its result in the ac_cv_func_strtold variable.

— Macro: AC_FUNC_STRNLEN

If the strnlen function is not available, or is buggy (like the one
from AIX 4.3), require an AC_LIBOBJ replacement for it.

This macro caches its result in the ac_cv_func_strnlen_working
variable.

This macro is obsolescent, as all current systems have a utime
that behaves this way. New programs need not use this macro.

— Macro: AC_FUNC_VPRINTF

If vprintf is found, define HAVE_VPRINTF. Otherwise, if
_doprnt is found, define HAVE_DOPRNT. (If vprintf
is available, you may assume that vfprintf and vsprintf
are also available.)

This macro is obsolescent, as all current systems have vprintf.
New programs need not use this macro.

— Macro: AC_REPLACE_FNMATCH

If the fnmatch function does not conform to Posix (see
AC_FUNC_FNMATCH), ask for its AC_LIBOBJ replacement.

The files fnmatch.c, fnmatch_loop.c, and fnmatch_.h
in the AC_LIBOBJ replacement directory are assumed to contain a
copy of the source code of GNU fnmatch. If necessary,
this source code is compiled as an AC_LIBOBJ replacement, and the
fnmatch_.h file is linked to fnmatch.h so that it can be
included in place of the system <fnmatch.h>.

This macro caches its result in the ac_cv_func_fnmatch_works
variable.

This macro is obsolescent, as it assumes the use of particular source
files. New programs should use Gnulib's fnmatch-posix module,
which provides this macro along with the source files. See Gnulib.

5.5.3 Generic Function Checks

These macros are used to find functions not covered by the “particular”
test macros. If the functions might be in libraries other than the
default C library, first call AC_CHECK_LIB for those libraries.
If you need to check the behavior of a function as well as find out
whether it is present, you have to write your own test for
it (see Writing Tests).

If C function function is available, run shell commands
action-if-found, otherwise action-if-not-found. If you just
want to define a symbol if the function is available, consider using
AC_CHECK_FUNCS instead. This macro checks for functions with C
linkage even when AC_LANG(C++) has been called, since C is more
standardized than C++. (see Language Choice, for more information
about selecting the language for checks.)

For each function enumerated in the blank-or-newline-separated argument
list, define HAVE_function (in all capitals) if it is available.
If action-if-found is given, it is additional shell code to
execute when one of the functions is found. You can give it a value of
‘break’ to break out of the loop on the first match. If
action-if-not-found is given, it is executed when one of the
functions is not found.

Results are cached for each function as in AC_CHECK_FUNC.

— Macro: AC_CHECK_FUNCS_ONCE (function...)

For each function enumerated in the blank-or-newline-separated argument
list, define HAVE_function (in all capitals) if it is available.
This is a once-only variant of AC_CHECK_FUNCS. It generates the
checking code at most once, so that configure is smaller and
faster; but the checks cannot be conditionalized and are always done once,
early during the configure run.

Autoconf follows a philosophy that was formed over the years by those
who have struggled for portability: isolate the portability issues in
specific files, and then program as if you were in a Posix
environment. Some functions may be missing or unfixable, and your
package must be ready to replace them.

Suitable replacements for many such problem functions are available from
Gnulib (see Gnulib).

— Macro: AC_LIBOBJ (function)

Specify that ‘function.c’ must be included in the executables
to replace a missing or broken implementation of function.

Technically, it adds ‘function.$ac_objext’ to the output
variable LIBOBJS if it is not already in, and calls
AC_LIBSOURCE for ‘function.c’. You should not
directly change LIBOBJS, since this is not traceable.

— Macro: AC_LIBSOURCE (file)

Specify that file might be needed to compile the project. If you
need to know what files might be needed by a configure.ac, you
should trace AC_LIBSOURCE. file must be a literal.

This macro is called automatically from AC_LIBOBJ, but you must
call it explicitly if you pass a shell variable to AC_LIBOBJ. In
that case, since shell variables cannot be traced statically, you must
pass to AC_LIBSOURCE any possible files that the shell variable
might cause AC_LIBOBJ to need. For example, if you want to pass
a variable $foo_or_bar to AC_LIBOBJ that holds either
"foo" or "bar", you should do:

AC_LIBSOURCE([foo.c])
AC_LIBSOURCE([bar.c])
AC_LIBOBJ([$foo_or_bar])

There is usually a way to avoid this, however, and you are encouraged to
simply call AC_LIBOBJ with literal arguments.

Note that this macro replaces the obsolete AC_LIBOBJ_DECL, with
slightly different semantics: the old macro took the function name,
e.g., foo, as its argument rather than the file name.

— Macro: AC_LIBSOURCES (files)

Like AC_LIBSOURCE, but accepts one or more files in a
comma-separated M4 list. Thus, the above example might be rewritten:

AC_LIBSOURCES([foo.c, bar.c])
AC_LIBOBJ([$foo_or_bar])

— Macro: AC_CONFIG_LIBOBJ_DIR (directory)

Specify that AC_LIBOBJ replacement files are to be found in
directory, a name relative to the top level of the
source tree. The replacement directory defaults to ., the top
level directory, and the most typical value is lib, corresponding
to ‘AC_CONFIG_LIBOBJ_DIR([lib])’.

configure might need to know the replacement directory for the
following reasons: (i) some checks use the replacement files, (ii) some
macros bypass broken system headers by installing links to the
replacement headers (iii) when used in conjunction with Automake,
within each makefile, directory is used as a relative path
from $(top_srcdir) to each object named in LIBOBJS and
LTLIBOBJS, etc.

It is common to merely check for the existence of a function, and ask
for its AC_LIBOBJ replacement if missing. The following macro is
a convenient shorthand.

— Macro: AC_REPLACE_FUNCS (function...)

Like AC_CHECK_FUNCS, but uses ‘AC_LIBOBJ(function)’ as
action-if-not-found. You can declare your replacement function by
enclosing the prototype in ‘#ifndef HAVE_function’. If the
system has the function, it probably declares it in a header file you
should be including, so you shouldn't redeclare it lest your declaration
conflict.

5.6 Header Files

The following macros check for the presence of certain C header files.
If there is no macro specifically defined to check for a header file you need,
and you don't need to check for any special properties of
it, then you can use one of the general header-file check macros.

5.6.1 Portability of Headers

This section documents some collected knowledge about common headers,
and the problems they cause. By definition, this list always requires
additions. A much more complete list is maintained by the Gnulib
project (see Gnulib), covering Posix Headers and Glibc Headers. Please help us keep
the gnulib list as complete as possible.

The C99 standard says that inttypes.h includes
stdint.h, so there's no need to include stdint.h
separately in a standard environment. Some implementations have
inttypes.h but not stdint.h (e.g., Solaris 7), but we don't
know of any implementation that has stdint.h but not
inttypes.h.

linux/irda.h

It requires linux/types.h and sys/socket.h.

linux/random.h

It requires linux/types.h.

net/if.h

On Darwin, this file requires that sys/socket.h be included
beforehand. One should run:

5.6.2 Particular Header Checks

These macros check for particular system header files—whether they
exist, and in some cases whether they declare certain symbols.

— Macro: AC_CHECK_HEADER_STDBOOL

Check whether stdbool.h exists and conforms to C99, and cache the
result in the ac_cv_header_stdbool_h variable. If the type
_Bool is defined, define HAVE__BOOL to 1.

This macro is intended for use by Gnulib (see Gnulib) and other
packages that supply a substitute stdbool.h on platforms lacking
a conforming one. The AC_HEADER_STDBOOL macro is better for code
that explicitly checks for stdbool.h.

— Macro: AC_HEADER_ASSERT

Check whether to enable assertions in the style of assert.h.
Assertions are enabled by default, but the user can override this by
invoking configure with the --disable-assert option.

— Macro: AC_HEADER_DIRENT

Check for the following header files. For the first one that is
found and defines ‘DIR’, define the listed C preprocessor macro:

dirent.h

HAVE_DIRENT_H

sys/ndir.h

HAVE_SYS_NDIR_H

sys/dir.h

HAVE_SYS_DIR_H

ndir.h

HAVE_NDIR_H

The directory-library declarations in your source code should look
something like the following:

Using the above declarations, the program would declare variables to be
of type struct dirent, not struct direct, and would access
the length of a directory entry name by passing a pointer to a
struct dirent to the NAMLEN macro.

This macro also checks for the SCO Xenix dir and x libraries.

This macro is obsolescent, as all current systems with directory
libraries have <dirent.h>. New programs need not use this macro.

If the macros S_ISDIR, S_ISREG, etc. defined in
sys/stat.h do not work properly (returning false positives),
define STAT_MACROS_BROKEN. This is the case on Tektronix UTekV,
Amdahl UTS and Motorola System V/88.

This macro is obsolescent, as no current systems have the bug.
New programs need not use this macro.

— Macro: AC_HEADER_STDBOOL

If stdbool.h exists and conforms to C99, define
HAVE_STDBOOL_H to 1; if the type _Bool is defined, define
HAVE__BOOL to 1. To fulfill the C99 requirements, your
program could contain the following code:

Alternatively you can use the ‘stdbool’ package of Gnulib
(see Gnulib). It simplifies your code so that it can say just
#include <stdbool.h>, and it adds support for less-common
platforms.

This macro caches its result in the ac_cv_header_stdbool_h
variable.

This macro differs from AC_CHECK_HEADER_STDBOOL only in that it
defines HAVE_STDBOOL_H whereas AC_CHECK_HEADER_STDBOOL
does not.

— Macro: AC_HEADER_STDC

Define STDC_HEADERS if the system has C header files
conforming to ANSI C89 (ISO C90).
Specifically, this macro checks for stdlib.h, stdarg.h,
string.h, and float.h; if the system has those, it
probably has the rest of the C89 header files. This macro also
checks whether string.h declares memchr (and thus
presumably the other mem functions), whether stdlib.h
declare free (and thus presumably malloc and other related
functions), and whether the ctype.h macros work on characters
with the high bit set, as the C standard requires.

If you use this macro, your code can refer to STDC_HEADERS to
determine whether the system has conforming header files (and probably C
library functions).

This macro caches its result in the ac_cv_header_stdc variable.

This macro is obsolescent, as current systems have conforming header
files. New programs need not use this macro.

Nowadays string.h is part of the C standard and declares functions like
strcpy, and strings.h is standardized by Posix and declares
BSD functions like bcopy; but
historically, string functions were a major sticking point in this area.
If you still want to worry about portability to ancient systems without
standard headers, there is so much variation
that it is probably easier to declare the functions you use than to
figure out exactly what the system header files declare. Some ancient systems
contained a mix of functions from the C standard and from BSD;
some were mostly standard but lacked ‘memmove’; some defined the
BSD functions as macros in string.h or
strings.h; some had only the BSD functions but
string.h; some declared the memory functions in memory.h,
some in string.h; etc. It is probably sufficient to check for
one string function and one memory function; if the library had the
standard versions of those then it probably had most of the others.
If you put the following in configure.ac:

# This example is obsolescent.
# Nowadays you can omit these macro calls.
AC_HEADER_STDC
AC_CHECK_FUNCS([strchr memcpy])

If you use a function like memchr, memset, strtok,
or strspn, which have no BSD equivalent, then macros don't
suffice to port to ancient hosts; you must provide an implementation of
each function. An easy
way to incorporate your implementations only when needed (since the ones
in system C libraries may be hand optimized) is to, taking memchr
for example, put it in memchr.c and use
‘AC_REPLACE_FUNCS([memchr])’.

— Macro: AC_HEADER_SYS_WAIT

If sys/wait.h exists and is compatible with Posix, define
HAVE_SYS_WAIT_H. Incompatibility can occur if sys/wait.h
does not exist, or if it uses the old BSD union wait instead
of int to store a status value. If sys/wait.h is not
Posix compatible, then instead of including it, define the
Posix macros with their usual interpretations. Here is an
example:

If a program may include both time.h and sys/time.h,
define TIME_WITH_SYS_TIME. On some ancient systems,
sys/time.h included time.h, but time.h was not
protected against multiple inclusion, so programs could not explicitly
include both files. This macro is useful in programs that use, for
example, struct timeval as well as
struct tm. It is best used in conjunction with
HAVE_SYS_TIME_H, which can be checked for using
AC_CHECK_HEADERS([sys/time.h]).

5.6.3 Generic Header Checks

These macros are used to find system header files not covered by the
“particular” test macros. If you need to check the contents of a header
as well as find out whether it is present, you have to write your own
test for it (see Writing Tests).

If the system header file header-file is compilable, execute shell
commands action-if-found, otherwise execute
action-if-not-found. If you just want to define a symbol if the
header file is available, consider using AC_CHECK_HEADERS
instead.

includes is decoded to determine the appropriate include
directives. If omitted or empty, configure will check for both header
existence (with the preprocessor) and usability (with the compiler),
using AC_INCLUDES_DEFAULT for the compile test. If
there is a discrepancy between the results, a warning is issued to the
user, and the compiler results are favored (see Present But Cannot Be Compiled). In general, favoring the compiler results means
that a header will be treated as not found even though the file exists,
because you did not provide enough prerequisites.

Providing a non-empty includes argument allows the code to provide
any prerequisites prior to including the header under test; it is common
to use the argument AC_INCLUDES_DEFAULT (see Default Includes). With an explicit fourth argument, no preprocessor test is
needed. As a special case, an includes of exactly ‘-’
triggers the older preprocessor check, which merely determines existence
of the file in the preprocessor search path; this should only be used as
a last resort (it is safer to determine the actual prerequisites and
perform a compiler check, or else use AC_PREPROC_IFELSE to make
it obvious that only a preprocessor check is desired).

This macro caches its result in the ac_cv_header_header-file
variable, with characters not suitable for a variable name mapped to
underscores.

For each given system header file header-file in the
blank-separated argument list that exists, define
HAVE_header-file (in all capitals). If action-if-found
is given, it is additional shell code to execute when one of the header
files is found. You can give it a value of ‘break’ to break out of
the loop on the first match. If action-if-not-found is given, it
is executed when one of the header files is not found.

includes is interpreted as in AC_CHECK_HEADER, in order to
choose the set of preprocessor directives supplied before the header
under test.

This macro caches its result in the ac_cv_header_header-file
variable, with characters not suitable for a variable name mapped to
underscores.

Previous versions of Autoconf merely checked whether the header was
accepted by the preprocessor. This was changed because the old test was
inappropriate for typical uses. Headers are typically used to compile,
not merely to preprocess, and the old behavior sometimes accepted
headers that clashed at compile-time (see Present But Cannot Be Compiled). If you need to check whether a header is preprocessable,
you can use AC_PREPROC_IFELSE (see Running the Preprocessor).

Actually requiring a header to compile improves the robustness of the
test, but it also requires
that you make sure that headers that must be included before the
header-file be part of the includes, (see Default Includes). If looking for bar.h, which requires that
foo.h be included before if it exists, we suggest the following
scheme:

The following variant generates smaller, faster configure
files if you do not need the full power of AC_CHECK_HEADERS.

— Macro: AC_CHECK_HEADERS_ONCE (header-file...)

For each given system header file header-file in the
blank-separated argument list that exists, define
HAVE_header-file (in all capitals).
This is a once-only variant of AC_CHECK_HEADERS. It generates the
checking code at most once, so that configure is smaller and
faster; but the checks cannot be conditionalized and are always done once,
early during the configure run. Thus, this macro is only safe
for checking headers that do not have prerequisites beyond what
AC_INCLUDES_DEFAULT provides.

5.7 Declarations

The following macros check for the declaration of variables and
functions. If there is no macro specifically defined to check for a
symbol you need, then you can use the general macros (see Generic Declarations) or, for more complex tests, you may use
AC_COMPILE_IFELSE (see Running the Compiler).

5.7.2 Generic Declaration Checks

If symbol (a function, variable, or constant) is not declared in
includes and a declaration is needed, run the shell commands
action-if-not-found, otherwise action-if-found.
includes is a series of include directives, defaulting to
AC_INCLUDES_DEFAULT (see Default Includes), which are used
prior to the declaration under test.

This macro actually tests whether symbol is defined as a macro or
can be used as an r-value, not whether it is really declared, because it
is much safer to avoid introducing extra declarations when they are not
needed. In order to facilitate use of C++ and overloaded function
declarations, it is possible to specify function argument types in
parentheses for types which can be zero-initialized:

AC_CHECK_DECL([basename(char *)])

This macro caches its result in the ac_cv_have_decl_symbol
variable, with characters not suitable for a variable name mapped to
underscores.

For each of the symbols (comma-separated list with optional
function argument types for C++ overloads), define
HAVE_DECL_symbol (in all capitals) to ‘1’ if
symbol is declared, otherwise to ‘0’. If
action-if-not-found is given, it is additional shell code to
execute when one of the function declarations is needed, otherwise
action-if-found is executed.

includes is a series of include directives, defaulting to
AC_INCLUDES_DEFAULT (see Default Includes), which are used
prior to the declarations under test.

Unlike the other ‘AC_CHECK_*S’ macros, when a symbol is not
declared, HAVE_DECL_symbol is defined to ‘0’ instead
of leaving HAVE_DECL_symbol undeclared. When you are
sure that the check was performed, use
HAVE_DECL_symbol in #if:

#if !HAVE_DECL_SYMBOL
extern char *symbol;
#endif

If the test may have not been performed, however, because it is safer
not to declare a symbol than to use a declaration that conflicts
with the system's one, you should use:

You fall into the second category only in extreme situations: either
your files may be used without being configured, or they are used during
the configuration. In most cases the traditional approach is enough.

This macro caches its results in ac_cv_have_decl_symbol
variables, with characters not suitable for a variable name mapped to
underscores.

— Macro: AC_CHECK_DECLS_ONCE (symbols)

For each of the symbols (comma-separated list), define
HAVE_DECL_symbol (in all capitals) to ‘1’ if
symbol is declared in the default include files, otherwise to
‘0’. This is a once-only variant of AC_CHECK_DECLS. It
generates the checking code at most once, so that configure is
smaller and faster; but the checks cannot be conditionalized and are
always done once, early during the configure run.

5.8 Structures

The following macros check for the presence of certain members in C
structures. If there is no macro specifically defined to check for a
member you need, then you can use the general structure-member macros
(see Generic Structures) or, for more complex tests, you may use
AC_COMPILE_IFELSE (see Running the Compiler).

HAVE_STRUCT_DIRENT_D_INO indicates only the presence of
d_ino, not whether its contents are always reliable.
Traditionally, a zero d_ino indicated a deleted directory entry,
though current systems hide this detail from the user and never return
zero d_ino values.
Many current systems report an incorrect d_ino for a directory
entry that is a mount point.

If struct stat contains an st_blocks member, define
HAVE_STRUCT_STAT_ST_BLOCKS. Otherwise, require an
AC_LIBOBJ replacement of ‘fileblocks’. The former name,
HAVE_ST_BLOCKS is to be avoided, as its support will cease in the
future.

This macro caches its result in the ac_cv_member_struct_stat_st_blocks
variable.

— Macro: AC_STRUCT_TM

If time.h does not define struct tm, define
TM_IN_SYS_TIME, which means that including sys/time.h
had better define struct tm.

This macro is obsolescent, as time.h defines struct tm in
current systems. New programs need not use this macro.

— Macro: AC_STRUCT_TIMEZONE

Figure out how to get the current timezone. If struct tm has a
tm_zone member, define HAVE_STRUCT_TM_TM_ZONE (and the
obsoleted HAVE_TM_ZONE). Otherwise, if the external array
tzname is found, define HAVE_TZNAME; if it is declared,
define HAVE_DECL_TZNAME.

Check for the existence of each ‘aggregate.member’ of
members using the previous macro. When member belongs to
aggregate, define HAVE_aggregate_member (in all
capitals, with spaces and dots replaced by underscores). If
action-if-found is given, it is executed for each of the found
members. If action-if-not-found is given, it is executed for each
of the members that could not be found.

includes is a series of include directives, defaulting to
AC_INCLUDES_DEFAULT (see Default Includes), which are used
prior to the members under test.

5.9 Types

The following macros check for C types, either builtin or typedefs. If
there is no macro specifically defined to check for a type you need, and
you don't need to check for any special properties of it, then you can
use a general type-check macro.

5.9.1 Particular Type Checks

These macros check for particular C types in sys/types.h,
stdlib.h, stdint.h, inttypes.h and others, if they
exist.

The Gnulib stdint module is an alternate way to define many of
these symbols; it is useful if you prefer your code to assume a
C99-or-better environment. See Gnulib.

— Macro: AC_TYPE_GETGROUPS

Define GETGROUPS_T to be whichever of gid_t or int
is the base type of the array argument to getgroups.

This macro caches the base type in the ac_cv_type_getgroups
variable.

— Macro: AC_TYPE_INT8_T

If stdint.h or inttypes.h does not define the type
int8_t, define int8_t to a signed
integer type that is exactly 8 bits wide and that uses two's complement
representation, if such a type exists.
If you are worried about porting to hosts that lack such a type, you can
use the results of this macro in C89-or-later code as follows:

If stdint.h or inttypes.h defines the type intmax_t,
define HAVE_INTMAX_T. Otherwise, define intmax_t to the
widest signed integer type.

— Macro: AC_TYPE_INTPTR_T

If stdint.h or inttypes.h defines the type intptr_t,
define HAVE_INTPTR_T. Otherwise, define intptr_t to a
signed integer type wide enough to hold a pointer, if such a type
exists.

— Macro: AC_TYPE_LONG_DOUBLE

If the C compiler supports a working long double type, define
HAVE_LONG_DOUBLE. The long double type might have the
same range and precision as double.

This macro caches its result in the ac_cv_type_long_double
variable.

This macro is obsolescent, as current C compilers support long
double. New programs need not use this macro.

— Macro: AC_TYPE_LONG_DOUBLE_WIDER

If the C compiler supports a working long double type with more
range or precision than the double type, define
HAVE_LONG_DOUBLE_WIDER.

This macro caches its result in the ac_cv_type_long_double_wider
variable.

— Macro: AC_TYPE_LONG_LONG_INT

If the C compiler supports a working long long int type, define
HAVE_LONG_LONG_INT. However, this test does not test
long long int values in preprocessor #if expressions,
because too many compilers mishandle such expressions.
See Preprocessor Arithmetic.

This macro caches its result in the ac_cv_type_long_long_int
variable.

— Macro: AC_TYPE_MBSTATE_T

Define HAVE_MBSTATE_T if <wchar.h> declares the
mbstate_t type. Also, define mbstate_t to be a type if
<wchar.h> does not declare it.

This macro caches its result in the ac_cv_type_mbstate_t
variable.

— Macro: AC_TYPE_MODE_T

Define mode_t to a suitable type, if standard headers do not
define it.

This macro caches its result in the ac_cv_type_mode_t variable.

— Macro: AC_TYPE_OFF_T

Define off_t to a suitable type, if standard headers do not
define it.

This macro caches its result in the ac_cv_type_off_t variable.

— Macro: AC_TYPE_PID_T

Define pid_t to a suitable type, if standard headers do not
define it.

This macro caches its result in the ac_cv_type_pid_t variable.

— Macro: AC_TYPE_SIZE_T

Define size_t to a suitable type, if standard headers do not
define it.

This macro caches its result in the ac_cv_type_size_t variable.

— Macro: AC_TYPE_SSIZE_T

Define ssize_t to a suitable type, if standard headers do not
define it.

This macro caches its result in the ac_cv_type_ssize_t variable.

— Macro: AC_TYPE_UID_T

Define uid_t and gid_t to suitable types, if standard
headers do not define them.

This macro caches its result in the ac_cv_type_uid_t variable.

— Macro: AC_TYPE_UINT8_T

If stdint.h or inttypes.h does not define the type
uint8_t, define uint8_t to an
unsigned integer type that is exactly 8 bits wide, if such a type
exists.
This is like AC_TYPE_INT8_T, except for unsigned integers.

If stdint.h or inttypes.h defines the type uintptr_t,
define HAVE_UINTPTR_T. Otherwise, define uintptr_t to an
unsigned integer type wide enough to hold a pointer, if such a type
exists.

— Macro: AC_TYPE_UNSIGNED_LONG_LONG_INT

If the C compiler supports a working unsigned long long int type,
define HAVE_UNSIGNED_LONG_LONG_INT. However, this test does not test
unsigned long long int values in preprocessor #if expressions,
because too many compilers mishandle such expressions.
See Preprocessor Arithmetic.

This macro caches its result in the ac_cv_type_unsigned_long_long_int
variable.

5.9.2 Generic Type Checks

Check whether type is defined. It may be a compiler builtin type
or defined by the includes. includes is a series of include
directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the type under test.

In C, type must be a type-name, so that the expression ‘sizeof
(type)’ is valid (but ‘sizeof ((type))’ is not). The
same test is applied when compiling for C++, which means that in C++
type should be a type-id and should not be an anonymous
‘struct’ or ‘union’.

This macro caches its result in the ac_cv_type_type
variable, with ‘*’ mapped to ‘p’ and other characters not
suitable for a variable name mapped to underscores.

For each type of the types that is defined, define
HAVE_type (in all capitals). Each type must follow
the rules of AC_CHECK_TYPE. If no includes are
specified, the default includes are used (see Default Includes). If
action-if-found is given, it is additional shell code to execute
when one of the types is found. If action-if-not-found is given,
it is executed when one of the types is not found.

Autoconf, up to 2.13, used to provide to another version of
AC_CHECK_TYPE, broken by design. In order to keep backward
compatibility, a simple heuristic, quite safe but not totally, is
implemented. In case of doubt, read the documentation of the former
AC_CHECK_TYPE, see Obsolete Macros.

5.10 Compilers and Preprocessors

All the tests for compilers (AC_PROG_CC, AC_PROG_CXX,
AC_PROG_F77) define the output variable EXEEXT based on
the output of the compiler, typically to the empty string if
Posix and ‘.exe’ if a DOS variant.

They also define the output variable OBJEXT based on the
output of the compiler, after .c files have been excluded, typically
to ‘o’ if Posix, ‘obj’ if a DOS variant.

If the compiler being used does not produce executables, the tests fail. If
the executables can't be run, and cross-compilation is not enabled, they
fail too. See Manual Configuration, for more on support for cross
compiling.

5.10.1 Specific Compiler Characteristics

Some compilers exhibit different behaviors.

Static/Dynamic Expressions

Autoconf relies on a trick to extract one bit of information from the C
compiler: using negative array sizes. For instance the following
excerpt of a C source demonstrates how to test whether ‘int’ objects are 4
bytes wide:

static int test_array[sizeof (int) == 4 ? 1 : -1];

To our knowledge, there is a single compiler that does not support this
trick: the HP C compilers (the real ones, not only the
“bundled”) on HP-UX 11.00.
They incorrectly reject the above program with the diagnostic
“Variable-length arrays cannot have static storage.”
This bug comes from HP compilers' mishandling of sizeof (int),
not from the ? 1 : -1, and
Autoconf works around this problem by casting sizeof (int) to
long int before comparing it.

5.10.2 Generic Compiler Characteristics

Define SIZEOF_type-or-expr (see Standard Symbols) to be
the size in bytes of type-or-expr, which may be either a type or
an expression returning a value that has a size. If the expression
‘sizeof (type-or-expr)’ is invalid, the result is 0.
includes is a series of include directives, defaulting to
AC_INCLUDES_DEFAULT (see Default Includes), which are used
prior to the expression under test.

This macro now works even when cross-compiling. The unused
argument was used when cross-compiling.

For example, the call

AC_CHECK_SIZEOF([int *])

defines SIZEOF_INT_P to be 8 on DEC Alpha AXP systems.

This macro caches its result in the ac_cv_sizeof_type-or-expr
variable, with ‘*’ mapped to ‘p’ and other characters not
suitable for a variable name mapped to underscores.

— Macro: AC_CHECK_ALIGNOF (type, [includes = ‘AC_INCLUDES_DEFAULT’])

Define ALIGNOF_type (see Standard Symbols) to be the
alignment in bytes of type. ‘type y;’ must be valid as
a structure member declaration. If ‘type’ is unknown, the result
is 0. If no includes are specified, the default includes are used
(see Default Includes).

This macro caches its result in the ac_cv_alignof_type-or-expr
variable, with ‘*’ mapped to ‘p’ and other characters not
suitable for a variable name mapped to underscores.

Store into the shell variable var the value of the integer
expression. The
value should fit in an initializer in a C variable of type signed
long. To support cross compilation (in which case, the macro only works on
hosts that use twos-complement arithmetic), it should be possible to evaluate
the expression at compile-time. If no includes are specified, the
default includes are used (see Default Includes).

Execute action-if-fails if the value cannot be determined correctly.

— Macro: AC_LANG_WERROR

Normally Autoconf ignores warnings generated by the compiler, linker, and
preprocessor. If this macro is used, warnings count as fatal
errors for the current language. This macro is useful when the
results of configuration are used where warnings are unacceptable; for
instance, if parts of a program are built with the GCC
-Werror
option. If the whole program is built using -Werror it is
often simpler to put -Werror in the compiler flags (CFLAGS,
etc.).

— Macro: AC_OPENMP

OpenMP specifies extensions of C, C++,
and Fortran that simplify optimization of shared memory parallelism,
which is a common problem on multicore CPUs.

If the current language is C, the macro AC_OPENMP sets the
variable OPENMP_CFLAGS to the C compiler flags needed for
supporting OpenMP. OPENMP_CFLAGS is set to empty if the
compiler already supports OpenMP, if it has no way to activate OpenMP
support, or if the user rejects OpenMP support by invoking
‘configure’ with the ‘--disable-openmp’ option.

OPENMP_CFLAGS needs to be used when compiling programs, when
preprocessing program source, and when linking programs. Therefore you
need to add $(OPENMP_CFLAGS) to the CFLAGS of C programs
that use OpenMP. If you preprocess OpenMP-specific C code, you also
need to add $(OPENMP_CFLAGS) to CPPFLAGS. The presence of
OpenMP support is revealed at compile time by the preprocessor macro
_OPENMP.

Linking a program with OPENMP_CFLAGS typically adds one more
shared library to the program's dependencies, so its use is recommended
only on programs that actually require OpenMP.

If the current language is C++, AC_OPENMP sets the variable
OPENMP_CXXFLAGS, suitably for the C++ compiler. The same remarks
hold as for C.

If the current language is Fortran 77 or Fortran, AC_OPENMP sets
the variable OPENMP_FFLAGS or OPENMP_FCFLAGS,
respectively. Similar remarks as for C hold, except that
CPPFLAGS is not used for Fortran, and no preprocessor macro
signals OpenMP support.

For portability, it is best to avoid spaces between ‘#’ and
‘pragma omp’. That is, write ‘#pragma omp’, not
‘# pragma omp’. The Sun WorkShop 6.2 C compiler chokes on the
latter.

This macro caches its result in the ac_cv_prog_c_openmp,
ac_cv_prog_cxx_openmp, ac_cv_prog_f77_openmp, or
ac_cv_prog_fc_openmp variable, depending on the current language.

The IRIX C compiler does not fail when #error is preprocessed; it
simply emits a diagnostic and continues, exiting successfully. So,
instead of an error directive like #error "Unsupported word size"
it is more portable to use an invalid directive like #Unsupported
word size in Autoconf tests. In ordinary source code, #error is
OK, since installers with inadequate compilers like IRIX can simply
examine these compilers' diagnostic output.

Determine a C compiler to use. If CC is not already set in the
environment, check for gcc and cc, then for other C
compilers. Set output variable CC to the name of the compiler
found.

This macro may, however, be invoked with an optional first argument
which, if specified, must be a blank-separated list of C compilers to
search for. This just gives the user an opportunity to specify an
alternative search list for the C compiler. For example, if you didn't
like the default order, then you could invoke AC_PROG_CC like
this:

AC_PROG_CC([gcc cl cc])

If the C compiler does not handle function prototypes correctly by
default, try to add an option to output variable CC to make it
so. This macro tries various options that select standard-conformance
modes on various systems.

After calling this macro you can check whether the C compiler has been
set to accept ANSI C89 (ISO C90); if not, the shell
variable
ac_cv_prog_cc_c89 is set to ‘no’. See also
AC_C_PROTOTYPES below.

If using the GNU C compiler, set shell variable GCC to
‘yes’. If output variable CFLAGS was not already set, set
it to -g -O2 for the GNU C compiler (-O2 on systems
where GCC does not accept -g), or -g for
other compilers. If your package does not like this default, then it is
acceptable to insert the line ‘: ${CFLAGS=""}’ after AC_INIT
and before AC_PROG_CC to select an empty default instead.

Many Autoconf macros use a compiler, and thus call
‘AC_REQUIRE([AC_PROG_CC])’ to ensure that the compiler has been
determined before the body of the outermost AC_DEFUN macro.
Although AC_PROG_CC is safe to directly expand multiple times, it
performs certain checks (such as the proper value of EXEEXT) only
on the first invocation. Therefore, care must be used when invoking
this macro from within another macro rather than at the top level
(see Expanded Before Required).

— Macro: AC_PROG_CC_C_O

If the C compiler does not accept the -c and -o options
simultaneously, define NO_MINUS_C_MINUS_O. This macro actually
tests both the compiler found by AC_PROG_CC, and, if different,
the first cc in the path. The test fails if one fails. This
macro was created for GNU Make to choose the default C compilation
rule.

For the compiler compiler, this macro caches its result in the
ac_cv_prog_cc_compiler_c_o variable.

— Macro: AC_PROG_CPP

Set output variable CPP to a command that runs the
C preprocessor. If ‘$CC -E’ doesn't work, /lib/cpp is used.
It is only portable to run CPP on files with a .c
extension.

Some preprocessors don't indicate missing include files by the error
status. For such preprocessors an internal variable is set that causes
other macros to check the standard error from the preprocessor and
consider the test failed if any warnings have been reported.
For most preprocessors, though, warnings do not cause include-file
tests to fail unless AC_PROG_CPP_WERROR is also specified.

— Macro: AC_PROG_CPP_WERROR

This acts like AC_PROG_CPP, except it treats warnings from the
preprocessor as errors even if the preprocessor exit status indicates
success. This is useful for avoiding headers that generate mandatory
warnings, such as deprecation notices.

The following macros check for C compiler or machine architecture
features. To check for characteristics not listed here, use
AC_COMPILE_IFELSE (see Running the Compiler) or
AC_RUN_IFELSE (see Runtime).

— Macro: AC_PROG_CC_STDC

If the C compiler cannot compile ISO Standard C (currently
C99), try to add an option to output variable CC to make it work.
If the compiler does not support C99, fall back to supporting
ANSI C89 (ISO C90).

After calling this macro you can check whether the C compiler has been
set to accept Standard C; if not, the shell variable
ac_cv_prog_cc_stdc is set to ‘no’.

— Macro: AC_PROG_CC_C89

If the C compiler is not in ANSI C89 (ISO C90) mode by
default, try to add an option to output variable CC to make it
so. This macro tries various options that select ANSI C89 on
some system or another, preferring extended functionality modes over
strict conformance modes. It considers the compiler to be in
ANSI C89 mode if it handles function prototypes correctly.

After calling this macro you can check whether the C compiler has been
set to accept ANSI C89; if not, the shell variable
ac_cv_prog_cc_c89 is set to ‘no’.

This macro is called automatically by AC_PROG_CC.

— Macro: AC_PROG_CC_C99

If the C compiler is not in C99 mode by default, try to add an
option to output variable CC to make it so. This macro tries
various options that select C99 on some system or another, preferring
extended functionality modes over strict conformance modes. It
considers the compiler to be in C99 mode if it handles _Bool,
// comments, flexible array members, inline, signed and
unsigned long long int, mixed code and declarations, named
initialization of structs,
restrict, va_copy, varargs macros, variable declarations
in for loops, and variable length arrays.

After calling this macro you can check whether the C compiler has been
set to accept C99; if not, the shell variable
ac_cv_prog_cc_c99 is set to ‘no’.

— Macro: AC_C_BACKSLASH_A

Define ‘HAVE_C_BACKSLASH_A’ to 1 if the C compiler understands
‘\a’.

This macro is obsolescent, as current C compilers understand ‘\a’.
New programs need not use this macro.

If words are stored with the most significant byte first (like Motorola
and SPARC CPUs), execute action-if-true. If words are stored with
the least significant byte first (like Intel and VAX CPUs), execute
action-if-false.

This macro runs a test-case if endianness cannot be determined from the
system header files. When cross-compiling, the test-case is not run but
grep'ed for some magic values. action-if-unknown is executed if
the latter case fails to determine the byte sex of the host system.

In some cases a single run of a compiler can generate code for multiple
architectures. This can happen, for example, when generating Mac OS X
universal binary files, which work on both PowerPC and Intel
architectures. In this case, the different variants might be for
different architectures whose endiannesses differ. If
configure detects this, it executes action-if-universal
instead of action-if-unknown.

The default for action-if-true is to define
‘WORDS_BIGENDIAN’. The default for action-if-false is to do
nothing. The default for action-if-unknown is to
abort configure and tell the installer how to bypass this test.
And finally, the default for action-if-universal is to ensure that
‘WORDS_BIGENDIAN’ is defined if and only if a universal build is
detected and the current code is big-endian; this default works only if
autoheader is used (see autoheader Invocation).

If you use this macro without specifying action-if-universal, you
should also use AC_CONFIG_HEADERS; otherwise
‘WORDS_BIGENDIAN’ may be set incorrectly for Mac OS X universal
binary files.

— Macro: AC_C_CONST

If the C compiler does not fully support the const keyword,
define const to be empty. Some C compilers that do
not define __STDC__ do support const; some compilers that
define __STDC__ do not completely support const. Programs
can simply use const as if every C compiler supported it; for
those that don't, the makefile or configuration header file
defines it as empty.

Occasionally installers use a C++ compiler to compile C code, typically
because they lack a C compiler. This causes problems with const,
because C and C++ treat const differently. For example:

const int foo;

is valid in C but not in C++. These differences unfortunately cannot be
papered over by defining const to be empty.

If autoconf detects this situation, it leaves const alone,
as this generally yields better results in practice. However, using a
C++ compiler to compile C code is not recommended or supported, and
installers who run into trouble in this area should get a C compiler
like GCC to compile their C code.

This macro caches its result in the ac_cv_c_const variable.

This macro is obsolescent, as current C compilers support const.
New programs need not use this macro.

— Macro: AC_C_RESTRICT

If the C compiler recognizes a variant spelling for the restrict
keyword (__restrict, __restrict__, or _Restrict),
then define restrict to that; this is more likely to do the right
thing with compilers that support language variants where plain
restrict is not a keyword. Otherwise, if the C compiler
recognizes the restrict keyword, don't do anything.
Otherwise, define restrict to be empty.
Thus, programs may simply use restrict as if every C compiler
supported it; for those that do not, the makefile
or configuration header defines it away.

Although support in C++ for the restrict keyword is not
required, several C++ compilers do accept the keyword.
This macro works for them, too.

This macro caches ‘no’ in the ac_cv_c_restrict variable
if restrict is not supported, and a supported spelling otherwise.

— Macro: AC_C_VOLATILE

If the C compiler does not understand the keyword volatile,
define volatile to be empty. Programs can simply use
volatile as if every C compiler supported it; for those that do
not, the makefile or configuration header defines it as
empty.

If the correctness of your program depends on the semantics of
volatile, simply defining it to be empty does, in a sense, break
your code. However, given that the compiler does not support
volatile, you are at its mercy anyway. At least your
program compiles, when it wouldn't before.
See Volatile Objects, for more about volatile.

In general, the volatile keyword is a standard C feature, so
you might expect that volatile is available only when
__STDC__ is defined. However, Ultrix 4.3's native compiler does
support volatile, but does not define __STDC__.

This macro is obsolescent, as current C compilers support volatile.
New programs need not use this macro.

— Macro: AC_C_INLINE

If the C compiler supports the keyword inline, do nothing.
Otherwise define inline to __inline__ or __inline
if it accepts one of those, otherwise define inline to be empty.

These days, using this macro is not necessary. The same information can
be determined by this portable alternative, thus avoiding the use of
preprocessor macros in the namespace reserved for the implementation.

#include <limits.h>
#if CHAR_MIN == 0
# define CHAR_UNSIGNED 1
#endif

— Macro: AC_C_STRINGIZE

If the C preprocessor supports the stringizing operator, define
HAVE_STRINGIZE. The stringizing operator is ‘#’ and is
found in macros such as this:

#define x(y) #y

This macro is obsolescent, as current C compilers support the
stringizing operator. New programs need not use this macro.

— Macro: AC_C_FLEXIBLE_ARRAY_MEMBER

If the C compiler supports flexible array members, define
FLEXIBLE_ARRAY_MEMBER to nothing; otherwise define it to 1.
That way, a declaration like this:

struct s
{
size_t n_vals;
double val[FLEXIBLE_ARRAY_MEMBER];
};

will let applications use the “struct hack” even with compilers that
do not support flexible array members. To allocate and use such an
object, you can use code like this:

If the C compiler supports variable-length arrays, define
HAVE_C_VARARRAYS. A variable-length array is an array of automatic
storage duration whose length is determined at run time, when the array
is declared.

— Macro: AC_C_TYPEOF

If the C compiler supports GCC's typeof syntax either
directly or
through a different spelling of the keyword (e.g., __typeof__),
define HAVE_TYPEOF. If the support is available only through a
different spelling, define typeof to that spelling.

— Macro: AC_C_PROTOTYPES

If function prototypes are understood by the compiler (as determined by
AC_PROG_CC), define PROTOTYPES and __PROTOTYPES.
Defining __PROTOTYPES is for the benefit of
header files that cannot use macros that infringe on user name space.

This macro is obsolescent, as current C compilers support prototypes.
New programs need not use this macro.

— Macro: AC_PROG_GCC_TRADITIONAL

Add -traditional to output variable CC if using the
GNU C compiler and ioctl does not work properly without
-traditional. That usually happens when the fixed header files
have not been installed on an old system.

This macro is obsolescent, since current versions of the GNU C
compiler fix the header files automatically when installed.

5.10.4 C++ Compiler Characteristics

Determine a C++ compiler to use. Check whether the environment variable
CXX or CCC (in that order) is set; if so, then set output
variable CXX to its value.

Otherwise, if the macro is invoked without an argument, then search for
a C++ compiler under the likely names (first g++ and c++
then other names). If none of those checks succeed, then as a last
resort set CXX to g++.

This macro may, however, be invoked with an optional first argument
which, if specified, must be a blank-separated list of C++ compilers to
search for. This just gives the user an opportunity to specify an
alternative search list for the C++ compiler. For example, if you
didn't like the default order, then you could invoke AC_PROG_CXX
like this:

AC_PROG_CXX([gcc cl KCC CC cxx cc++ xlC aCC c++ g++])

If using the GNU C++ compiler, set shell variable GXX to
‘yes’. If output variable CXXFLAGS was not already set, set
it to -g -O2 for the GNU C++ compiler (-O2 on
systems where G++ does not accept -g), or -g for other
compilers. If your package does not like this default, then it is
acceptable to insert the line ‘: ${CXXFLAGS=""}’ after AC_INIT
and before AC_PROG_CXX to select an empty default instead.

— Macro: AC_PROG_CXXCPP

Set output variable CXXCPP to a command that runs the C++
preprocessor. If ‘$CXX -E’ doesn't work, /lib/cpp is used.
It is portable to run CXXCPP only on files with a .c,
.C, .cc, or .cpp extension.

Some preprocessors don't indicate missing include files by the error
status. For such preprocessors an internal variable is set that causes
other macros to check the standard error from the preprocessor and
consider the test failed if any warnings have been reported. However,
it is not known whether such broken preprocessors exist for C++.

— Macro: AC_PROG_CXX_C_O

Test whether the C++ compiler accepts the options -c and
-o simultaneously, and define CXX_NO_MINUS_C_MINUS_O,
if it does not.

5.10.5 Objective C Compiler Characteristics

Determine an Objective C compiler to use. If OBJC is not already
set in the environment, check for Objective C compilers. Set output
variable OBJC to the name of the compiler found.

This macro may, however, be invoked with an optional first argument
which, if specified, must be a blank-separated list of Objective C compilers to
search for. This just gives the user an opportunity to specify an
alternative search list for the Objective C compiler. For example, if you
didn't like the default order, then you could invoke AC_PROG_OBJC
like this:

AC_PROG_OBJC([gcc objcc objc])

If using the GNU Objective C compiler, set shell variable
GOBJC to ‘yes’. If output variable OBJCFLAGS was not
already set, set it to -g -O2 for the GNU Objective C
compiler (-O2 on systems where gcc does not accept
-g), or -g for other compilers.

— Macro: AC_PROG_OBJCPP

Set output variable OBJCPP to a command that runs the Objective C
preprocessor. If ‘$OBJC -E’ doesn't work, /lib/cpp is used.

5.10.6 Objective C++ Compiler Characteristics

Determine an Objective C++ compiler to use. If OBJCXX is not already
set in the environment, check for Objective C++ compilers. Set output
variable OBJCXX to the name of the compiler found.

This macro may, however, be invoked with an optional first argument
which, if specified, must be a blank-separated list of Objective C++ compilers
to search for. This just gives the user an opportunity to specify an
alternative search list for the Objective C++ compiler. For example, if you
didn't like the default order, then you could invoke AC_PROG_OBJCXX
like this:

AC_PROG_OBJCXX([gcc g++ objcc++ objcxx])

If using the GNU Objective C++ compiler, set shell variable
GOBJCXX to ‘yes’. If output variable OBJCXXFLAGS was not
already set, set it to -g -O2 for the GNU Objective C++
compiler (-O2 on systems where gcc does not accept
-g), or -g for other compilers.

— Macro: AC_PROG_OBJCXXCPP

Set output variable OBJCXXCPP to a command that runs the Objective C++
preprocessor. If ‘$OBJCXX -E’ doesn't work, /lib/cpp is used.

5.10.7 Erlang Compiler and Interpreter Characteristics

Autoconf defines the following macros for determining paths to the essential
Erlang/OTP programs:

— Macro: AC_ERLANG_PATH_ERLC ([value-if-not-found], [path = ‘$PATH’])

Determine an Erlang compiler to use. If ERLC is not already set in the
environment, check for erlc. Set output variable ERLC to the
complete path of the compiler command found. In addition, if ERLCFLAGS
is not set in the environment, set it to an empty value.

The two optional arguments have the same meaning as the two last arguments of
macro AC_PATH_PROG for looking for the erlc program. For
example, to look for erlc only in the /usr/lib/erlang/bin
directory:

AC_ERLANG_PATH_ERLC([not found], [/usr/lib/erlang/bin])

— Macro: AC_ERLANG_NEED_ERLC ([path = ‘$PATH’])

A simplified variant of the AC_ERLANG_PATH_ERLC macro, that prints an
error message and exits the configure script if the erlc
program is not found.

— Macro: AC_ERLANG_PATH_ERL ([value-if-not-found], [path = ‘$PATH’])

Determine an Erlang interpreter to use. If ERL is not already
set in the
environment, check for erl. Set output variable ERL to the
complete path of the interpreter command found.

The two optional arguments have the same meaning as the two last arguments of
macro AC_PATH_PROG for looking for the erl program. For
example, to look for erl only in the /usr/lib/erlang/bin
directory:

AC_ERLANG_PATH_ERL([not found], [/usr/lib/erlang/bin])

— Macro: AC_ERLANG_NEED_ERL ([path = ‘$PATH’])

A simplified variant of the AC_ERLANG_PATH_ERL macro, that prints an
error message and exits the configure script if the erl
program is not found.

5.10.8 Fortran Compiler Characteristics

The Autoconf Fortran support is divided into two categories: legacy
Fortran 77 macros (F77), and modern Fortran macros (FC).
The former are intended for traditional Fortran 77 code, and have output
variables like F77, FFLAGS, and FLIBS. The latter
are for newer programs that can (or must) compile under the newer
Fortran standards, and have output variables like FC,
FCFLAGS, and FCLIBS.

Except for the macros AC_FC_SRCEXT, AC_FC_FREEFORM,
AC_FC_FIXEDFORM, and AC_FC_LINE_LENGTH (see below), the
FC and F77 macros behave almost identically, and so they
are documented together in this section.

— Macro: AC_PROG_F77 ([compiler-search-list])

Determine a Fortran 77 compiler to use. If F77 is not already
set in the environment, then check for g77 and f77, and
then some other names. Set the output variable F77 to the name
of the compiler found.

This macro may, however, be invoked with an optional first argument
which, if specified, must be a blank-separated list of Fortran 77
compilers to search for. This just gives the user an opportunity to
specify an alternative search list for the Fortran 77 compiler. For
example, if you didn't like the default order, then you could invoke
AC_PROG_F77 like this:

AC_PROG_F77([fl32 f77 fort77 xlf g77 f90 xlf90])

If using g77 (the GNU Fortran 77 compiler), then
set the shell variable G77 to ‘yes’.
If the output variable FFLAGS was not already set in the
environment, then set it to -g -02 for g77 (or -O2
where g77 does not accept -g). Otherwise, set
FFLAGS to -g for all other Fortran 77 compilers.

The result of the GNU test is cached in the
ac_cv_f77_compiler_gnu variable, acceptance of -g in the
ac_cv_prog_f77_g variable.

— Macro: AC_PROG_FC ([compiler-search-list], [dialect])

Determine a Fortran compiler to use. If FC is not already set in
the environment, then dialect is a hint to indicate what Fortran
dialect to search for; the default is to search for the newest available
dialect. Set the output variable FC to the name of the compiler
found.

By default, newer dialects are preferred over older dialects, but if
dialect is specified then older dialects are preferred starting
with the specified dialect. dialect can currently be one of
Fortran 77, Fortran 90, or Fortran 95. However, this is only a hint of
which compiler name to prefer (e.g., f90 or f95),
and no attempt is made to guarantee that a particular language standard
is actually supported. Thus, it is preferable that you avoid the
dialect option, and use AC_PROG_FC only for code compatible with
the latest Fortran standard.

This macro may, alternatively, be invoked with an optional first argument
which, if specified, must be a blank-separated list of Fortran
compilers to search for, just as in AC_PROG_F77.

If using gfortran or g77 (the GNU Fortran compilers), then
set the shell variable GFC to ‘yes’.
If the output variable FCFLAGS was not already set in the
environment, then set it to -g -02 for GNU g77 (or
-O2 where g77 does not accept -g). Otherwise,
set FCFLAGS to -g for all other Fortran compilers.

The result of the GNU test is cached in the ac_cv_fc_compiler_gnu
variable, acceptance of -g in the ac_cv_prog_fc_g
variable.

— Macro: AC_PROG_F77_C_O
— Macro: AC_PROG_FC_C_O

Test whether the Fortran compiler accepts the options -c and
-o simultaneously, and define F77_NO_MINUS_C_MINUS_O or
FC_NO_MINUS_C_MINUS_O, respectively, if it does not.

The result of the test is cached in the ac_cv_prog_f77_c_o or
ac_cv_prog_fc_c_o variable, respectively.

The following macros check for Fortran compiler characteristics.
To check for characteristics not listed here, use
AC_COMPILE_IFELSE (see Running the Compiler) or
AC_RUN_IFELSE (see Runtime), making sure to first set the
current language to Fortran 77 or Fortran via AC_LANG([Fortran 77])
or AC_LANG(Fortran) (see Language Choice).

— Macro: AC_F77_LIBRARY_LDFLAGS
— Macro: AC_FC_LIBRARY_LDFLAGS

Determine the linker flags (e.g., -L and -l) for the
Fortran intrinsic and runtime libraries that are required to
successfully link a Fortran program or shared library. The output
variable FLIBS or FCLIBS is set to these flags (which
should be included after LIBS when linking).

This macro is intended to be used in those situations when it is
necessary to mix, e.g., C++ and Fortran source code in a single
program or shared library (see Mixing Fortran 77 With C and C++).

For example, if object files from a C++ and Fortran compiler must be
linked together, then the C++ compiler/linker must be used for linking
(since special C++-ish things need to happen at link time like calling
global constructors, instantiating templates, enabling exception
support, etc.).

However, the Fortran intrinsic and runtime libraries must be linked in
as well, but the C++ compiler/linker doesn't know by default how to add
these Fortran 77 libraries. Hence, this macro was created to determine
these Fortran libraries.

The macros AC_F77_DUMMY_MAIN and AC_FC_DUMMY_MAIN or
AC_F77_MAIN and AC_FC_MAIN are probably also necessary to
link C/C++ with Fortran; see below. Further, it is highly recommended
that you use AC_CONFIG_HEADERS (see Configuration Headers)
because the complex defines that the function wrapper macros create
may not work with C/C++ compiler drivers.

These macros internally compute the flag needed to verbose linking
output and cache it in ac_cv_prog_f77_v or ac_cv_prog_fc_v
variables, respectively. The computed linker flags are cached in
ac_cv_f77_libs or ac_cv_fc_libs, respectively.

With many compilers, the Fortran libraries detected by
AC_F77_LIBRARY_LDFLAGS or AC_FC_LIBRARY_LDFLAGS provide
their own main entry function that initializes things like
Fortran I/O, and which then calls a user-provided entry function named
(say) MAIN__ to run the user's program. The
AC_F77_DUMMY_MAIN and AC_FC_DUMMY_MAIN or
AC_F77_MAIN and AC_FC_MAIN macros figure out how to deal with
this interaction.

When using Fortran for purely numerical functions (no I/O, etc.) often
one prefers to provide one's own main and skip the Fortran
library initializations. In this case, however, one may still need to
provide a dummy MAIN__ routine in order to prevent linking errors
on some systems. AC_F77_DUMMY_MAIN or AC_FC_DUMMY_MAIN
detects whether any such routine is required for linking, and
what its name is; the shell variable F77_DUMMY_MAIN or
FC_DUMMY_MAIN holds this name, unknown when no solution
was found, and none when no such dummy main is needed.

By default, action-if-found defines F77_DUMMY_MAIN or
FC_DUMMY_MAIN to the name of this routine (e.g., MAIN__)
if it is required. action-if-not-found defaults to
exiting with an error.

In order to link with Fortran routines, the user's C/C++ program should
then include the following code to define the dummy main if it is
needed:

Note that this macro is called automatically from AC_F77_WRAPPERS
or AC_FC_WRAPPERS; there is generally no need to call it
explicitly unless one wants to change the default actions.

The result of this macro is cached in the ac_cv_f77_dummy_main or
ac_cv_fc_dummy_main variable, respectively.

— Macro: AC_F77_MAIN
— Macro: AC_FC_MAIN

As discussed above, many Fortran libraries allow you to provide an entry
point called (say) MAIN__ instead of the usual main, which
is then called by a main function in the Fortran libraries that
initializes things like Fortran I/O. The
AC_F77_MAIN and AC_FC_MAIN macros detect whether it is
possible to utilize such an alternate main function, and defines
F77_MAIN and FC_MAIN to the name of the function. (If no
alternate main function name is found, F77_MAIN and FC_MAIN are
simply defined to main.)

Thus, when calling Fortran routines from C that perform things like I/O,
one should use this macro and declare the "main" function like so:

The result of this macro is cached in the ac_cv_f77_main or
ac_cv_fc_main variable, respectively.

— Macro: AC_F77_WRAPPERS
— Macro: AC_FC_WRAPPERS

Defines C macros F77_FUNC (name, NAME), FC_FUNC (name, NAME),
F77_FUNC_(name, NAME), and FC_FUNC_(name, NAME) to properly
mangle the names of C/C++ identifiers, and identifiers with underscores,
respectively, so that they match the name-mangling scheme used by the
Fortran compiler.

Fortran is case-insensitive, and in order to achieve this the Fortran
compiler converts all identifiers into a canonical case and format. To
call a Fortran subroutine from C or to write a C function that is
callable from Fortran, the C program must explicitly use identifiers in
the format expected by the Fortran compiler. In order to do this, one
simply wraps all C identifiers in one of the macros provided by
AC_F77_WRAPPERS or AC_FC_WRAPPERS. For example, suppose
you have the following Fortran 77 subroutine:

Note that we pass both the lowercase and uppercase versions of the
function name to F77_FUNC so that it can select the right one.
Note also that all parameters to Fortran 77 routines are passed as
pointers (see Mixing Fortran 77 With C and C++).

(Replace F77 with FC for Fortran instead of Fortran 77.)

Although Autoconf tries to be intelligent about detecting the
name-mangling scheme of the Fortran compiler, there may be Fortran
compilers that it doesn't support yet. In this case, the above code
generates a compile-time error, but some other behavior
(e.g., disabling Fortran-related features) can be induced by checking
whether F77_FUNC or FC_FUNC is defined.

Now, to call that routine from a C program, we would do something like:

{
double x = 2.7183, y;
FOOBAR_F77 (&x, &y);
}

If the Fortran identifier contains an underscore (e.g., foo_bar),
you should use F77_FUNC_ or FC_FUNC_ instead of
F77_FUNC or FC_FUNC (with the same arguments). This is
because some Fortran compilers mangle names differently if they contain
an underscore.

The name mangling scheme is encoded in the ac_cv_f77_mangling or
ac_cv_fc_mangling cache variable, respectively, and also used for
the AC_F77_FUNC and AC_FC_FUNC macros described below.

Given an identifier name, set the shell variable shellvar to
hold the mangled version name according to the rules of the
Fortran linker (see also AC_F77_WRAPPERS or
AC_FC_WRAPPERS). shellvar is optional; if it is not
supplied, the shell variable is simply name. The purpose of
this macro is to give the caller a way to access the name-mangling
information other than through the C preprocessor as above, for example,
to call Fortran routines from some language other than C/C++.

By default, the FC macros perform their tests using a .f
extension for source-code files. Some compilers, however, only enable
newer language features for appropriately named files, e.g., Fortran 90
features only for .f90 files, or preprocessing only with
.F files or maybe other upper-case extensions. On the other
hand, some other compilers expect all source files to end in .f
and require special flags to support other file name extensions. The
AC_FC_SRCEXT and AC_FC_PP_SRCEXT macros deal with these
issues.

The AC_FC_SRCEXT macro tries to get the FC compiler to
accept files ending with the extension .ext (i.e.,
ext does not contain the dot). If any special compiler
flags are needed for this, it stores them in the output variable
FCFLAGS_ext. This extension and these flags are then used
for all subsequent FC tests (until AC_FC_SRCEXT or
AC_FC_PP_SRCEXT is called another time).

For example, you would use AC_FC_SRCEXT(f90) to employ the
.f90 extension in future tests, and it would set the
FCFLAGS_f90 output variable with any extra flags that are needed
to compile such files.

Similarly, the AC_FC_PP_SRCEXT macro tries to get the FC
compiler to preprocess and compile files with the extension
.ext. When both fpp and cpp style
preprocessing are provided, the former is preferred, as the latter may
treat continuation lines, // tokens, and white space differently
from what some Fortran dialects expect. Conversely, if you do not want
files to be preprocessed, use only lower-case characters in the file
name extension. Like with AC_FC_SRCEXT(f90), any needed flags
are stored in the FCFLAGS_ext variable.

The FCFLAGS_ext flags can not be simply absorbed
into FCFLAGS, for two reasons based on the limitations of some
compilers. First, only one FCFLAGS_ext can be used at a
time, so files with different extensions must be compiled separately.
Second, FCFLAGS_ext must appear immediately before
the source-code file name when compiling. So, continuing the example
above, you might compile a foo.f90 file in your makefile with the
command:

foo.o: foo.f90
$(FC) -c $(FCFLAGS) $(FCFLAGS_f90) '$(srcdir)/foo.f90'

If AC_FC_SRCEXT or AC_FC_PP_SRCEXT succeeds in compiling
files with the ext extension, it calls action-if-success
(defaults to nothing). If it fails, and cannot find a way to make the
FC compiler accept such files, it calls action-if-failure
(defaults to exiting with an error message).

The AC_FC_SRCEXT and AC_FC_PP_SRCEXT macros cache their
results in ac_cv_fc_srcext_ext and
ac_cv_fc_pp_srcext_ext variables, respectively.

Find a flag to specify defines for preprocessed Fortran. Not all
Fortran compilers use -D. Substitute FC_DEFINE with
the result and call action-if-success (defaults to nothing) if
successful, and action-if-failure (defaults to failing with an
error message) if not.

This macro calls AC_FC_PP_SRCEXT([F]) in order to learn how to
preprocess a conftest.F file, but restores a previously used
Fortran source file extension afterwards again.

Try to ensure that the Fortran compiler ($FC) allows free-format
source code (as opposed to the older fixed-format style from Fortran
77). If necessary, it may add some additional flags to FCFLAGS.

This macro is most important if you are using the default .f
extension, since many compilers interpret this extension as indicating
fixed-format source unless an additional flag is supplied. If you
specify a different extension with AC_FC_SRCEXT, such as
.f90, then AC_FC_FREEFORM ordinarily succeeds without
modifying FCFLAGS. For extensions which the compiler does not
know about, the flag set by the AC_FC_SRCEXT macro might let
the compiler assume Fortran 77 by default, however.

If AC_FC_FREEFORM succeeds in compiling free-form source, it
calls action-if-success (defaults to nothing). If it fails, it
calls action-if-failure (defaults to exiting with an error
message).

The result of this test, or ‘none’ or ‘unknown’, is cached in
the ac_cv_fc_freeform variable.

Try to ensure that the Fortran compiler ($FC) allows the old
fixed-format source code (as opposed to free-format style). If
necessary, it may add some additional flags to FCFLAGS.

This macro is needed for some compilers alias names like xlf95
which assume free-form source code by default, and in case you want to
use fixed-form source with an extension like .f90 which many
compilers interpret as free-form by default. If you specify a different
extension with AC_FC_SRCEXT, such as .f, then
AC_FC_FIXEDFORM ordinarily succeeds without modifying
FCFLAGS.

If AC_FC_FIXEDFORM succeeds in compiling fixed-form source, it
calls action-if-success (defaults to nothing). If it fails, it
calls action-if-failure (defaults to exiting with an error
message).

The result of this test, or ‘none’ or ‘unknown’, is cached in
the ac_cv_fc_fixedform variable.

Try to ensure that the Fortran compiler ($FC) accepts long source
code lines. The length argument may be given as 80, 132, or
unlimited, and defaults to 132. Note that line lengths above 254
columns are not portable, and some compilers do not accept more than 132
columns at least for fixed format source. If necessary, it may add some
additional flags to FCFLAGS.

If AC_FC_LINE_LENGTH succeeds in compiling fixed-form source, it
calls action-if-success (defaults to nothing). If it fails, it
calls action-if-failure (defaults to exiting with an error
message).

The result of this test, or ‘none’ or ‘unknown’, is cached in
the ac_cv_fc_line_length variable.

The AC_FC_CHECK_BOUNDS macro tries to enable array bounds checking
in the Fortran compiler. If successful, the action-if-success
is called and any needed flags are added to FCFLAGS. Otherwise,
action-if-failure is called, which defaults to failing with an error
message. The macro currently requires Fortran 90 or a newer dialect.

The result of the macro is cached in the ac_cv_fc_check_bounds
variable.

Try to disallow implicit declarations in the Fortran compiler. If
successful, action-if-success is called and any needed flags
are added to FFLAGS or FCFLAGS, respectively. Otherwise,
action-if-failure is called, which defaults to failing with an error
message.

The result of these macros are cached in the
ac_cv_f77_implicit_none and ac_cv_fc_implicit_none
variables, respectively.

— Macro: AC_FC_MODULE_EXTENSION

Find the Fortran 90 module file name extension. Most Fortran 90
compilers store module information in files separate from the object
files. The module files are usually named after the name of the module
rather than the source file name, with characters possibly turned to
upper case, plus an extension, often .mod.

Not all compilers use module files at all, or by default. The Cray
Fortran compiler requires -e m in order to store and search
module information in .mod files rather than in object files.
Likewise, the Fujitsu Fortran compilers uses the -Am option to
indicate how module information is stored.

The AC_FC_MODULE_EXTENSION macro computes the module extension
without the leading dot, and stores that in the FC_MODEXT
variable. If the compiler does not produce module files, or the
extension cannot be determined, FC_MODEXT is empty. Typically,
the result of this macro may be used in cleanup make rules as
follows:

clean-modules:
-test -z "$(FC_MODEXT)" || rm -f *.$(FC_MODEXT)

The extension, or ‘unknown’, is cached in the
ac_cv_fc_module_ext variable.

Find the compiler flag to include Fortran 90 module information from
another directory, and store that in the FC_MODINC variable.
Call action-if-success (defaults to nothing) if successful, and
set FC_MODINC to empty and call action-if-failure (defaults
to exiting with an error message) if not.

Most Fortran 90 compilers provide a way to specify module directories.
Some have separate flags for the directory to write module files to,
and directories to search them in, whereas others only allow writing to
the current directory or to the first directory specified in the include
path. Further, with some compilers, the module search path and the
preprocessor search path can only be modified with the same flag. Thus,
for portability, write module files to the current directory only and
list that as first directory in the search path.

There may be no whitespace between FC_MODINC and the following
directory name, but FC_MODINC may contain trailing white space.
For example, if you use Automake and would like to search ../lib
for module files, you can use the following:

The flag is cached in the ac_cv_fc_module_flag variable.
The substituted value of FC_MODINC may refer to the
ac_empty dummy placeholder empty variable, to avoid losing
the significant trailing whitespace in a Makefile.

Find the compiler flag to write Fortran 90 module information to
another directory, and store that in the FC_MODOUT variable.
Call action-if-success (defaults to nothing) if successful, and
set FC_MODOUT to empty and call action-if-failure (defaults
to exiting with an error message) if not.

Not all Fortran 90 compilers write module files, and of those that do,
not all allow writing to a directory other than the current one, nor
do all have separate flags for writing and reading; see the description
of AC_FC_MODULE_FLAG above. If you need to be able to write to
another directory, for maximum portability use FC_MODOUT before
any FC_MODINC and include both the current directory and the one
you write to in the search path:

AM_FCFLAGS = $(FC_MODOUT)../mod $(FC_MODINC)../mod $(FC_MODINC). ...

The flag is cached in the ac_cv_fc_module_output_flag variable.
The substituted value of FC_MODOUT may refer to the
ac_empty dummy placeholder empty variable, to avoid losing
the significant trailing whitespace in a Makefile.

5.11 System Services

The following macros check for operating system services or capabilities.

— Macro: AC_PATH_X

Try to locate the X Window System include files and libraries. If the
user gave the command line options --x-includes=dir and
--x-libraries=dir, use those directories.

If either or both were not given, get the missing values by running
xmkmf (or an executable pointed to by the XMKMF
environment variable) on a trivial Imakefile and examining the
makefile that it produces. Setting XMKMF to ‘false’
disables this method.

If this method fails to find the X Window System, configure
looks for the files in several directories where they often reside.
If either method is successful, set the shell variables
x_includes and x_libraries to their locations, unless they
are in directories the compiler searches by default.

If both methods fail, or the user gave the command line option
--without-x, set the shell variable no_x to ‘yes’;
otherwise set it to the empty string.

— Macro: AC_PATH_XTRA

An enhanced version of AC_PATH_X. It adds the C compiler flags
that X needs to output variable X_CFLAGS, and the X linker flags
to X_LIBS. Define X_DISPLAY_MISSING if X is not
available.

This macro also checks for special libraries that some systems need in
order to compile X programs. It adds any that the system needs to
output variable X_EXTRA_LIBS. And it checks for special X11R6
libraries that need to be linked with before -lX11, and adds
any found to the output variable X_PRE_LIBS.

— Macro: AC_SYS_INTERPRETER

Check whether the system supports starting scripts with a line of the
form ‘#!/bin/sh’ to select the interpreter to use for the script.
After running this macro, shell code in configure.ac can check
the shell variable interpval; it is set to ‘yes’
if the system supports ‘#!’, ‘no’ if not.

— Macro: AC_SYS_LARGEFILE

Arrange for 64-bit file offsets, known as
large-file support. On some hosts, one must use special compiler
options to build programs that can access large files. Append any such
options to the output variable CC. Define
_FILE_OFFSET_BITS and _LARGE_FILES if necessary.

Large-file support can be disabled by configuring with the
--disable-largefile option.

If you use this macro, check that your program works even when
off_t is wider than long int, since this is common when
large-file support is enabled. For example, it is not correct to print
an arbitrary off_t value X with printf ("%ld",
(long int) X).

The LFS introduced the fseeko and ftello functions to
replace their C counterparts fseek and ftell that do not
use off_t. Take care to use AC_FUNC_FSEEKO to make their
prototypes available when using them and large-file support is
enabled.

— Macro: AC_SYS_LONG_FILE_NAMES

If the system supports file names longer than 14 characters, define
HAVE_LONG_FILE_NAMES.

— Macro: AC_SYS_POSIX_TERMIOS

Check to see if the Posix termios headers and functions are available on the
system. If so, set the shell variable ac_cv_sys_posix_termios to
‘yes’. If not, set the variable to ‘no’.

5.12 Posix Variants

The following macro makes it possible to use features of Posix that are
extensions to C, as well as platform extensions not defined by Posix.

— Macro: AC_USE_SYSTEM_EXTENSIONS

This macro was introduced in Autoconf 2.60. If possible, enable
extensions to C or Posix on hosts that normally disable the extensions,
typically due to standards-conformance namespace issues. This should be
called before any macros that run the C compiler. The following
preprocessor macros are defined where appropriate:

_GNU_SOURCE

Enable extensions on GNU/Linux.

__EXTENSIONS__

Enable general extensions on Solaris.

_POSIX_PTHREAD_SEMANTICS

Enable threading extensions on Solaris.

_TANDEM_SOURCE

Enable extensions for the HP NonStop platform.

_ALL_SOURCE

Enable extensions for AIX 3, and for Interix.

_POSIX_SOURCE

Enable Posix functions for Minix.

_POSIX_1_SOURCE

Enable additional Posix functions for Minix.

_MINIX

Identify Minix platform. This particular preprocessor macro is
obsolescent, and may be removed in a future release of Autoconf.

5.13 Erlang Libraries

The following macros check for an installation of Erlang/OTP, and for the
presence of certain Erlang libraries. All those macros require the
configuration of an Erlang interpreter and an Erlang compiler
(see Erlang Compiler and Interpreter).

— Macro: AC_ERLANG_SUBST_ERTS_VER

Set the output variable ERLANG_ERTS_VER to the version of the
Erlang runtime system (as returned by Erlang's
erlang:system_info(version) function). The result of this test
is cached if caching is enabled when running configure. The
ERLANG_ERTS_VER variable is not intended to be used for testing
for features of specific ERTS versions, but to be used for substituting
the ERTS version in Erlang/OTP release resource files (.rel
files), as shown below.

— Macro: AC_ERLANG_SUBST_ROOT_DIR

Set the output variable ERLANG_ROOT_DIR to the path to the base
directory in which Erlang/OTP is installed (as returned by Erlang's
code:root_dir/0 function). The result of this test is cached if
caching is enabled when running configure.

— Macro: AC_ERLANG_SUBST_LIB_DIR

Set the output variable ERLANG_LIB_DIR to the path of the library
directory of Erlang/OTP (as returned by Erlang's
code:lib_dir/0 function), which subdirectories each contain an installed
Erlang/OTP library. The result of this test is cached if caching is enabled
when running configure.

Test whether the Erlang/OTP library library is installed by
calling Erlang's code:lib_dir/1 function. The result of this
test is cached if caching is enabled when running configure.
action-if-found is a list of shell commands to run if the library
is installed; action-if-not-found is a list of shell commands to
run if it is not. Additionally, if the library is installed, the output
variable ‘ERLANG_LIB_DIR_library’ is set to the path to the
library installation directory, and the output variable
‘ERLANG_LIB_VER_library’ is set to the version number that is
part of the subdirectory name, if it is in the standard form
(library-version). If the directory name does not
have a version part, ‘ERLANG_LIB_VER_library’ is set to the
empty string. If the library is not installed,
‘ERLANG_LIB_DIR_library’ and
‘ERLANG_LIB_VER_library’ are set to "not found". For
example, to check if library stdlib is installed:

The ‘ERLANG_LIB_VER_library’ variables (set by
AC_ERLANG_CHECK_LIB) and the ERLANG_ERTS_VER variable (set
by AC_ERLANG_SUBST_ERTS_VER) are not intended to be used for
testing for features of specific versions of libraries or of the Erlang
runtime system. Those variables are intended to be substituted in
Erlang release resource files (.rel files). For instance, to
generate a example.rel file for an application depending on the
stdlib library, configure.ac could contain:

In addition to the above macros, which test installed Erlang libraries, the
following macros determine the paths to the directories into which newly built
Erlang libraries are to be installed:

— Macro: AC_ERLANG_SUBST_INSTALL_LIB_DIR

Set the ERLANG_INSTALL_LIB_DIR output variable to the directory into
which every built Erlang library should be installed in a separate
subdirectory.
If this variable is not set in the environment when configure runs,
its default value is ${libdir}/erlang/lib.

— Macro: AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR (library, version)

Set the ‘ERLANG_INSTALL_LIB_DIR_library’ output variable to the
directory into which the built Erlang library library version
version should be installed. If this variable is not set in the
environment when configure runs, its default value is
‘$ERLANG_INSTALL_LIB_DIR/library-version’, the value of the
ERLANG_INSTALL_LIB_DIR variable being set by the
AC_ERLANG_SUBST_INSTALL_LIB_DIR macro.

6 Writing Tests

If the existing feature tests don't do something you need, you have to
write new ones. These macros are the building blocks. They provide
ways for other macros to check whether various kinds of features are
available and report the results.

This chapter contains some suggestions and some of the reasons why the
existing tests are written the way they are. You can also learn a lot
about how to write Autoconf tests by looking at the existing ones. If
something goes wrong in one or more of the Autoconf tests, this
information can help you understand the assumptions behind them, which
might help you figure out how to best solve the problem.

These macros check the output of the compiler system of the current
language (see Language Choice). They do not cache the results of
their tests for future use (see Caching Results), because they don't
know enough about the information they are checking for to generate a
cache variable name. They also do not print any messages, for the same
reason. The checks for particular kinds of features call these macros
and do cache their results and print messages about what they're
checking for.

When you write a feature test that could be applicable to more than one
software package, the best thing to do is encapsulate it in a new macro.
See Writing Autoconf Macros, for how to do that.

6.1 Language Choice

Autoconf-generated configure scripts check for the C compiler and
its features by default. Packages that use other programming languages
(maybe more than one, e.g., C and C++) need to test features of the
compilers for the respective languages. The following macros determine
which programming language is used in the subsequent tests in
configure.ac.

— Macro: AC_LANG (language)

Do compilation tests using the compiler, preprocessor, and file
extensions for the specified language.

Supported languages are:

‘C’

Do compilation tests using CC and CPP and use extension
.c for test programs. Use compilation flags: CPPFLAGS with
CPP, and both CPPFLAGS and CFLAGS with CC.

‘C++’

Do compilation tests using CXX and CXXCPP and use
extension .C for test programs. Use compilation flags:
CPPFLAGS with CXXCPP, and both CPPFLAGS and
CXXFLAGS with CXX.

‘Fortran 77’

Do compilation tests using F77 and use extension .f for
test programs. Use compilation flags: FFLAGS.

‘Fortran’

Do compilation tests using FC and use extension .f (or
whatever has been set by AC_FC_SRCEXT) for test programs. Use
compilation flags: FCFLAGS.

‘Erlang’

Compile and execute tests using ERLC and ERL and use extension
.erl for test Erlang modules. Use compilation flags: ERLCFLAGS.

‘Objective C’

Do compilation tests using OBJC and OBJCPP and use
extension .m for test programs. Use compilation flags:
CPPFLAGS with OBJCPP, and both CPPFLAGS and
OBJCFLAGS with OBJC.

‘Objective C++’

Do compilation tests using OBJCXX and OBJCXXCPP and use
extension .mm for test programs. Use compilation flags:
CPPFLAGS with OBJCXXCPP, and both CPPFLAGS and
OBJCXXFLAGS with OBJCXX.

‘Go’

Do compilation tests using GOC and use extension .go for
test programs. Use compilation flags GOFLAGS.

— Macro: AC_LANG_PUSH (language)

Remember the current language (as set by AC_LANG) on a stack, and
then select the language. Use this macro and AC_LANG_POP
in macros that need to temporarily switch to a particular language.

— Macro: AC_LANG_POP ([language])

Select the language that is saved on the top of the stack, as set by
AC_LANG_PUSH, and remove it from the stack.

If given, language specifies the language we just quit. It
is a good idea to specify it when it's known (which should be the
case...), since Autoconf detects inconsistencies.

Check statically that the current language is language.
You should use this in your language specific macros
to avoid that they be called with an inappropriate language.

This macro runs only at autoconf time, and incurs no cost at
configure time. Sadly enough and because Autoconf is a two
layer language 2, the macros
AC_LANG_PUSH and AC_LANG_POP cannot be “optimizing”,
therefore as much as possible you ought to avoid using them to wrap
your code, rather, require from the user to run the macro with a
correct current language, and check it with AC_LANG_ASSERT.
And anyway, that may help the user understand she is running a Fortran
macro while expecting a result about her Fortran 77 compiler...

— Macro: AC_REQUIRE_CPP

Ensure that whichever preprocessor would currently be used for tests has
been found. Calls AC_REQUIRE (see Prerequisite Macros) with an
argument of either AC_PROG_CPP or AC_PROG_CXXCPP,
depending on which language is current.

6.2.1 Guidelines for Test Programs

This motto means that testing samples must be written with the same
strictness as real programs are written. In particular, you should
avoid “shortcuts” and simplifications.

Don't just play with the preprocessor if you want to prepare a
compilation. For instance, using cpp to check whether a header is
functional might let your configure accept a header which
causes some compiler error. Do not hesitate to check a header with
other headers included before, especially required headers.

Make sure the symbols you use are properly defined, i.e., refrain from
simply declaring a function yourself instead of including the proper
header.

Test programs should not write to standard output. They
should exit with status 0 if the test succeeds, and with status 1
otherwise, so that success
can be distinguished easily from a core dump or other failure;
segmentation violations and other failures produce a nonzero exit
status. Unless you arrange for exit to be declared, test
programs should return, not exit, from main,
because on many systems exit is not declared by default.

Test programs can use #if or #ifdef to check the values of
preprocessor macros defined by tests that have already run. For
example, if you call AC_HEADER_STDBOOL, then later on in
configure.ac you can have a test program that includes
stdbool.h conditionally:

#ifdef HAVE_STDBOOL_H
# include <stdbool.h>
#endif

Both #if HAVE_STDBOOL_H and #ifdef HAVE_STDBOOL_H will
work with any standard C compiler. Some developers prefer #if
because it is easier to read, while others prefer #ifdef because
it avoids diagnostics with picky compilers like GCC with the
-Wundef option.

If a test program needs to use or create a data file, give it a name
that starts with conftest, such as conftest.data. The
configure script cleans up by running ‘rm -f -r conftest*’
after running test programs and if the script is interrupted.

6.2.2 Test Functions

These days it's safe to assume support for function prototypes
(introduced in C89).

Functions that test programs declare should also be conditionalized for
C++, which requires ‘extern "C"’ prototypes. Make sure to not
include any header files containing clashing prototypes.

#ifdef __cplusplus
extern "C"
#endif
void *valloc (size_t);

If a test program calls a function with invalid parameters (just to see
whether it exists), organize the program to ensure that it never invokes
that function. You can do this by calling it in another function that is
never invoked. You can't do it by putting it after a call to
exit, because GCC version 2 knows that exit
never returns
and optimizes out any code that follows it in the same block.

If you include any header files, be sure to call the functions
relevant to them with the correct number of arguments, even if they are
just 0, to avoid compilation errors due to prototypes. GCC
version 2
has internal prototypes for several functions that it automatically
inlines; for example, memcpy. To avoid errors when checking for
them, either pass them the correct number of arguments or redeclare them
with a different return type (such as char).

6.2.3 Generating Sources

Autoconf provides a set of macros that can be used to generate test
source files. They are written to be language generic, i.e., they
actually depend on the current language (see Language Choice) to
“format” the output properly.

— Macro: AC_LANG_CONFTEST (source)

Save the source text in the current test source file:
conftest.extension where the extension depends on the
current language. As of Autoconf 2.63b, the source file also contains
the results of all of the AC_DEFINE performed so far.

Note that the source is evaluated exactly once, like regular
Autoconf macro arguments, and therefore (i) you may pass a macro
invocation, (ii) if not, be sure to double quote if needed.

This macro issues a warning during autoconf processing if
source does not include an expansion of the macro
AC_LANG_DEFINES_PROVIDED (note that both AC_LANG_SOURCE and
AC_LANG_PROGRAM call this macro, and thus avoid the warning).

This macro is seldom called directly, but is used under the hood by more
common macros such as AC_COMPILE_IFELSE and AC_RUN_IFELSE.

— Macro: AC_LANG_DEFINES_PROVIDED

This macro is called as a witness that the file
conftest.extension appropriate for the current language is
complete, including all previously determined results from
AC_DEFINE. This macro is seldom called directly, but exists if
you have a compelling reason to write a conftest file without using
AC_LANG_SOURCE, yet still want to avoid a syntax warning from
AC_LANG_CONFTEST.

— Macro: AC_LANG_SOURCE (source)

Expands into the source, with the definition of
all the AC_DEFINE performed so far. This macro includes an
expansion of AC_LANG_DEFINES_PROVIDED.

In many cases, you may find it more convenient to use the wrapper
AC_LANG_PROGRAM.

In Erlang tests, the created source file is that of an Erlang module called
conftest (conftest.erl). This module defines and exports
at least
one start/0 function, which is called to perform the test. The
prologue is optional code that is inserted between the module header and
the start/0 function definition. body is the body of the
start/0 function without the final period (see Runtime, about
constraints on this function's behavior).

Expands into a source file which consists of the prologue, and
then a call to the function as body of the main function (e.g.,
main in C). Since it uses AC_LANG_PROGRAM, the feature
of the latter are available.

This function will probably be replaced in the future by a version
which would enable specifying the arguments. The use of this macro is
not encouraged, as it violates strongly the typing system.

This macro cannot be used for Erlang tests.

— Macro: AC_LANG_FUNC_LINK_TRY (function)

Expands into a source file which uses the function in the body of
the main function (e.g., main in C). Since it uses
AC_LANG_PROGRAM, the features of the latter are available.

As AC_LANG_CALL, this macro is documented only for completeness.
It is considered to be severely broken, and in the future will be
removed in favor of actual function calls (with properly typed
arguments).

6.3 Running the Preprocessor

Sometimes one might need to run the preprocessor on some source file.
Usually it is a bad idea, as you typically need to compile
your project, not merely run the preprocessor on it; therefore you
certainly want to run the compiler, not the preprocessor. Resist the
temptation of following the easiest path.

Nevertheless, if you need to run the preprocessor, then use
AC_PREPROC_IFELSE.

The macros described in this section cannot be used for tests in Erlang,
Fortran, or Go, since those languages require no preprocessor.

Run the preprocessor of the current language (see Language Choice)
on the input, run the shell commands action-if-true on
success, action-if-false otherwise. The input can be made
by AC_LANG_PROGRAM and friends.

This macro uses CPPFLAGS, but not CFLAGS, because
-g, -O, etc. are not valid options to many C
preprocessors.

It is customary to report unexpected failures with
AC_MSG_FAILURE. If needed, action-if-true can further access
the preprocessed output in the file conftest.i.

The macro AC_TRY_CPP (see Obsolete Macros) used to play the
role of AC_PREPROC_IFELSE, but double quotes its argument, making
it impossible to use it to elaborate sources. You are encouraged to
get rid of your old use of the macro AC_TRY_CPP in favor of
AC_PREPROC_IFELSE, but, in the first place, are you sure you need
to run the preprocessor and not the compiler?

program is the text of a C or C++ program, on which shell
variable, back quote, and backslash substitutions are performed. If the
output of running the preprocessor on program matches the
extended regular expression pattern, execute shell commands
action-if-found, otherwise execute action-if-not-found.

6.4 Running the Compiler

To check for a syntax feature of the current language's (see Language Choice) compiler, such as whether it recognizes a certain keyword, or
simply to try some library feature, use AC_COMPILE_IFELSE to try
to compile a small program that uses that feature.

Run the compiler and compilation flags of the current language
(see Language Choice) on the input, run the shell commands
action-if-true on success, action-if-false otherwise. The
input can be made by AC_LANG_PROGRAM and friends.

It is customary to report unexpected failures with
AC_MSG_FAILURE. This macro does not try to link; use
AC_LINK_IFELSE if you need to do that (see Running the Linker). If needed, action-if-true can further access the
just-compiled object file conftest.$OBJEXT.

This macro uses AC_REQUIRE for the compiler associated with the
current language, which means that if the compiler has not yet been
determined, the compiler determination will be made prior to the body of
the outermost AC_DEFUN macro that triggered this macro to
expand (see Expanded Before Required).

For tests in Erlang, the input must be the source code of a module named
conftest. AC_COMPILE_IFELSE generates a conftest.beam
file that can be interpreted by the Erlang virtual machine (ERL). It is
recommended to use AC_LANG_PROGRAM to specify the test program,
to ensure that the Erlang module has the right name.

6.5 Running the Linker

To check for a library, a function, or a global variable, Autoconf
configure scripts try to compile and link a small program that
uses it. This is unlike Metaconfig, which by default uses nm or
ar on the C library to try to figure out which functions are
available. Trying to link with the function is usually a more reliable
approach because it avoids dealing with the variations in the options
and output formats of nm and ar and in the location of the
standard libraries. It also allows configuring for cross-compilation or
checking a function's runtime behavior if needed. On the other hand,
it can be slower than scanning the libraries once, but accuracy is more
important than speed.

AC_LINK_IFELSE is used to compile test programs to test for
functions and global variables. It is also used by AC_CHECK_LIB
to check for libraries (see Libraries), by adding the library being
checked for to LIBS temporarily and trying to link a small
program.

— Macro: AC_LINK_IFELSE (input, [action-if-true], [action-if-false])

Run the compiler (and compilation flags) and the linker of the current
language (see Language Choice) on the input, run the shell
commands action-if-true on success, action-if-false
otherwise. The input can be made by AC_LANG_PROGRAM and
friends. If needed, action-if-true can further access the
just-linked program file conftest$EXEEXT.

LDFLAGS and LIBS are used for linking, in addition to the
current compilation flags.

It is customary to report unexpected failures with
AC_MSG_FAILURE. This macro does not try to execute the program;
use AC_RUN_IFELSE if you need to do that (see Runtime).

The AC_LINK_IFELSE macro cannot be used for Erlang tests, since Erlang
programs are interpreted and do not require linking.

6.6 Checking Runtime Behavior

Sometimes you need to find out how a system performs at runtime, such
as whether a given function has a certain capability or bug. If you
can, make such checks when your program runs instead of when it is
configured. You can check for things like the machine's endianness when
your program initializes itself.

If you really need to test for a runtime behavior while configuring,
you can write a test program to determine the result, and compile and
run it using AC_RUN_IFELSE. Avoid running test programs if
possible, because this prevents people from configuring your package for
cross-compiling.

Run the compiler (and compilation flags) and the linker of the current
language (see Language Choice) on the input, then execute the
resulting program. If the program returns an exit
status of 0 when executed, run shell commands action-if-true.
Otherwise, run shell commands action-if-false.

The input can be made by AC_LANG_PROGRAM and friends.
LDFLAGS and LIBS are used for linking, in addition to the
compilation flags of the current language (see Language Choice).
Additionally, action-if-true can run ./conftest$EXEEXT
for further testing.

In the action-if-false section, the failing exit status is
available in the shell variable ‘$?’. This exit status might be
that of a failed compilation, or it might be that of a failed program
execution.

If cross-compilation mode is enabled (this is the case if either the
compiler being used does not produce executables that run on the system
where configure is being run, or if the options --build
and --host were both specified and their values are different),
then the test program is
not run. If the optional shell commands action-if-cross-compiling
are given, those commands are run instead; typically these commands
provide pessimistic defaults that allow cross-compilation to work even
if the guess was wrong. If the fourth argument is empty or omitted, but
cross-compilation is detected, then configure prints an error
message and exits. If you want your package to be useful in a
cross-compilation scenario, you should provide a non-empty
action-if-cross-compiling clause, as well as wrap the
AC_RUN_IFELSE compilation inside an AC_CACHE_CHECK
(see Caching Results) which allows the user to override the
pessimistic default if needed.

It is customary to report unexpected failures with
AC_MSG_FAILURE.

autoconf prints a warning message when creating
configure each time it encounters a call to
AC_RUN_IFELSE with no action-if-cross-compiling argument
given. If you are not concerned about users configuring your package
for cross-compilation, you may ignore the warning. A few of the macros
distributed with Autoconf produce this warning message; but if this is a
problem for you, please report it as a bug, along with an appropriate
pessimistic guess to use instead.

To configure for cross-compiling you can also choose a value for those
parameters based on the canonical system name (see Manual Configuration). Alternatively, set up a test results cache file with
the correct values for the host system (see Caching Results).

To provide a default for calls of AC_RUN_IFELSE that are embedded
in other macros, including a few of the ones that come with Autoconf,
you can test whether the shell variable cross_compiling is set to
‘yes’, and then use an alternate method to get the results instead
of calling the macros.

It is also permissible to temporarily assign to cross_compiling
in order to force tests to behave as though they are in a
cross-compilation environment, particularly since this provides a way to
test your action-if-cross-compiling even when you are not using a
cross-compiler.

# We temporarily set cross-compile mode to force AC_COMPUTE_INT
# to use the slow link-only method
save_cross_compiling=$cross_compiling
cross_compiling=yes
AC_COMPUTE_INT([...])
cross_compiling=$save_cross_compiling

Erlang tests must exit themselves the Erlang VM by calling the halt/1
function: the given status code is used to determine the success of the test
(status is 0) or its failure (status is different than 0), as
explained above. It must be noted that data output through the standard output
(e.g., using io:format/2) may be truncated when halting the VM.
Therefore, if a test must output configuration information, it is recommended
to create and to output data into the temporary file named conftest.out,
using the functions of module file. The conftest.out file is
automatically deleted by the AC_RUN_IFELSE macro. For instance, a
simplified implementation of Autoconf's AC_ERLANG_SUBST_LIB_DIR
macro is:

Darwin is also known as Mac OS X. Beware that the file system can be
case-preserving, but case insensitive. This can cause nasty problems,
since for instance the installation attempt for a package having an
INSTALL file can result in ‘make install’ report that
nothing was to be done!

That's all dependent on whether the file system is a UFS (case
sensitive) or HFS+ (case preserving). By default Apple wants you to
install the OS on HFS+. Unfortunately, there are some pieces of
software which really need to be built on UFS. We may want to rebuild
Darwin to have both UFS and HFS+ available (and put the /local/build
tree on the UFS).

QNX 4.25

QNX is a realtime operating system running on Intel architecture
meant to be scalable from the small embedded systems to the hundred
processor super-computer. It claims to be Posix certified. More
information is available on the
QNX home page.

Officially this was called the “Seventh Edition” of “the UNIX
time-sharing system” but we use the more-common name “Unix version 7”.
Documentation is available in the
Unix Seventh Edition Manual.
Previous versions of Unix are called “Unix version 6”, etc., but
they were not as widely used.

6.8 Multiple Cases

Some operations are accomplished in several possible ways, depending on
the OS variant. Checking for them essentially requires a “case
statement”. Autoconf does not directly provide one; however, it is
easy to simulate by using a shell variable to keep track of whether a
way to perform the operation has been found yet.

Here is an example that uses the shell variable fstype to keep
track of whether the remaining cases need to be checked. Note that
since the value of fstype is under our control, we don't have to
use the longer ‘test "x$fstype" = xno’.

7 Results of Tests

Once configure has determined whether a feature exists, what can
it do to record that information? There are four sorts of things it can
do: define a C preprocessor symbol, set a variable in the output files,
save the result in a cache file for future configure runs, and
print a message letting the user know the result of the test.

7.1 Defining C Preprocessor Symbols

A common action to take in response to a feature test is to define a C
preprocessor symbol indicating the results of the test. That is done by
calling AC_DEFINE or AC_DEFINE_UNQUOTED.

By default, AC_OUTPUT places the symbols defined by these macros
into the output variable DEFS, which contains an option
-Dsymbol=value for each symbol defined. Unlike in
Autoconf version 1, there is no variable DEFS defined while
configure is running. To check whether Autoconf macros have
already defined a certain C preprocessor symbol, test the value of the
appropriate cache variable, as in this example:

If AC_CONFIG_HEADERS has been called, then instead of creating
DEFS, AC_OUTPUT creates a header file by substituting the
correct values into #define statements in a template file.
See Configuration Headers, for more information about this kind of
output.

Define variable to value (verbatim), by defining a C
preprocessor macro for variable. variable should be a C
identifier, optionally suffixed by a parenthesized argument list to
define a C preprocessor macro with arguments. The macro argument list,
if present, should be a comma-separated list of C identifiers, possibly
terminated by an ellipsis ‘...’ if C99 syntax is employed.
variable should not contain comments, white space, trigraphs,
backslash-newlines, universal character names, or non-ASCII
characters.

value may contain backslash-escaped newlines, which will be
preserved if you use AC_CONFIG_HEADERS but flattened if passed
via @DEFS@ (with no effect on the compilation, since the
preprocessor sees only one line in the first place). value should
not contain raw newlines. If you are not using
AC_CONFIG_HEADERS, value should not contain any ‘#’
characters, as make tends to eat them. To use a shell
variable, use AC_DEFINE_UNQUOTED instead.

description is only useful if you are using
AC_CONFIG_HEADERS. In this case, description is put into
the generated config.h.in as the comment before the macro define.
The following example defines the C preprocessor variable
EQUATION to be the string constant ‘"$a > $b"’:

AC_DEFINE([EQUATION], ["$a > $b"],
[Equation string.])

If neither value nor description are given, then
value defaults to 1 instead of to the empty string. This is for
backwards compatibility with older versions of Autoconf, but this usage
is obsolescent and may be withdrawn in future versions of Autoconf.

If the variable is a literal string, it is passed to
m4_pattern_allow (see Forbidden Patterns).

If multiple AC_DEFINE statements are executed for the same
variable name (not counting any parenthesized argument list),
the last one wins.

Like AC_DEFINE, but three shell expansions are
performed—once—on variable and value: variable expansion
(‘$’), command substitution (‘`’), and backslash escaping
(‘\’), as if in an unquoted here-document. Single and double quote
characters in the value have no
special meaning. Use this macro instead of AC_DEFINE when
variable or value is a shell variable. Examples:

Due to a syntactical bizarreness of the Bourne shell, do not use
semicolons to separate AC_DEFINE or AC_DEFINE_UNQUOTED
calls from other macro calls or shell code; that can cause syntax errors
in the resulting configure script. Use either blanks or
newlines. That is, do this:

7.2 Setting Output Variables

Another way to record the results of tests is to set output
variables, which are shell variables whose values are substituted into
files that configure outputs. The two macros below create new
output variables. See Preset Output Variables, for a list of output
variables that are always available.

— Macro: AC_SUBST (variable, [value])

Create an output variable from a shell variable. Make AC_OUTPUT
substitute the variable variable into output files (typically one
or more makefiles). This means that AC_OUTPUT
replaces instances of ‘@variable@’ in input files with the
value that the shell variable variable has when AC_OUTPUT
is called. The value can contain any non-NUL character, including
newline. If you are using Automake 1.11 or newer, for newlines in values
you might want to consider using AM_SUBST_NOTMAKE to prevent
automake from adding a line variable =
@variable@ to the Makefile.in files (see Automake).

Variable occurrences should not overlap: e.g., an input file should
not contain ‘@var1@var2@’ if var1 and var2
are variable names.
The substituted value is not rescanned for more output variables;
occurrences of ‘@variable@’ in the value are inserted
literally into the output file. (The algorithm uses the special marker
|#_!!_#| internally, so neither the substituted value nor the
output file may contain |#_!!_#|.)

Another way to create an output variable from a shell variable. Make
AC_OUTPUT insert (without substitutions) the contents of the file
named by shell variable variable into output files. This means
that AC_OUTPUT replaces instances of
‘@variable@’ in output files (such as Makefile.in)
with the contents of the file that the shell variable variable
names when AC_OUTPUT is called. Set the variable to
/dev/null for cases that do not have a file to insert.
This substitution occurs only when the ‘@variable@’ is on a
line by itself, optionally surrounded by spaces and tabs. The
substitution replaces the whole line, including the spaces, tabs, and
the terminating newline.

This macro is useful for inserting makefile fragments containing
special dependencies or other make directives for particular host
or target types into makefiles. For example, configure.ac
could contain:

Running configure in varying environments can be extremely
dangerous. If for instance the user runs ‘CC=bizarre-cc
./configure’, then the cache, config.h, and many other output
files depend upon bizarre-cc being the C compiler. If
for some reason the user runs ./configure again, or if it is
run via ‘./config.status --recheck’, (See Automatic Remaking,
and see config.status Invocation), then the configuration can be
inconsistent, composed of results depending upon two different
compilers.

Environment variables that affect this situation, such as ‘CC’
above, are called precious variables, and can be declared as such
by AC_ARG_VAR.

— Macro: AC_ARG_VAR (variable, description)

Declare variable is a precious variable, and include its
description in the variable section of ‘./configure --help’.

Being precious means that

variable is substituted via AC_SUBST.

The value of variable when configure was launched is
saved in the cache, including if it was not specified on the command
line but via the environment. Indeed, while configure can
notice the definition of CC in ‘./configure CC=bizarre-cc’,
it is impossible to notice it in ‘CC=bizarre-cc ./configure’,
which, unfortunately, is what most users do.

We emphasize that it is the initial value of variable which
is saved, not that found during the execution of configure.
Indeed, specifying ‘./configure FOO=foo’ and letting
‘./configure’ guess that FOO is foo can be two
different things.

variable is checked for consistency between two
configure runs. For instance:

$ ./configure --silent --config-cache
$ CC=cc ./configure --silent --config-cache
configure: error: `CC' was not set in the previous run
configure: error: changes in the environment can compromise \
the build
configure: error: run `make distclean' and/or \
`rm config.cache' and start over

and similarly if the variable is unset, or if its content is changed.
If the content has white space changes only, then the error is degraded
to a warning only, but the old value is reused.

variable is kept during automatic reconfiguration
(see config.status Invocation) as if it had been passed as a command
line argument, including when no cache is used:

7.3 Special Characters in Output Variables

Many output variables are intended to be evaluated both by
make and by the shell. Some characters are expanded
differently in these two contexts, so to avoid confusion these
variables' values should not contain any of the following characters:

" # $ & ' ( ) * ; < > ? [ \ ^ ` |

Also, these variables' values should neither contain newlines, nor start
with ‘~’, nor contain white space or ‘:’ immediately followed
by ‘~’. The values can contain nonempty sequences of white space
characters like tabs and spaces, but each such sequence might
arbitrarily be replaced by a single space during substitution.

These restrictions apply both to the values that configure
computes, and to the values set directly by the user. For example, the
following invocations of configure are problematic, since they
attempt to use special characters within CPPFLAGS and white space
within $(srcdir):

7.4 Caching Results

To avoid checking for the same features repeatedly in various
configure scripts (or in repeated runs of one script),
configure can optionally save the results of many checks in a
cache file (see Cache Files). If a configure script
runs with caching enabled and finds a cache file, it reads the results
of previous runs from the cache and avoids rerunning those checks. As a
result, configure can then run much faster than if it had to
perform all of the checks every time.

— Macro: AC_CACHE_VAL (cache-id, commands-to-set-it)

Ensure that the results of the check identified by cache-id are
available. If the results of the check were in the cache file that was
read, and configure was not given the --quiet or
--silent option, print a message saying that the result was
cached; otherwise, run the shell commands commands-to-set-it. If
the shell commands are run to determine the value, the value is
saved in the cache file just before configure creates its output
files. See Cache Variable Names, for how to choose the name of the
cache-id variable.

The commands-to-set-itmust have no side effects except for
setting the variable cache-id, see below.

— Macro: AC_CACHE_CHECK (message, cache-id, commands-to-set-it)

A wrapper for AC_CACHE_VAL that takes care of printing the
messages. This macro provides a convenient shorthand for the most
common way to use these macros. It calls AC_MSG_CHECKING for
message, then AC_CACHE_VAL with the cache-id and
commands arguments, and AC_MSG_RESULT with cache-id.

The commands-to-set-itmust have no side effects except for
setting the variable cache-id, see below.

It is common to find buggy macros using AC_CACHE_VAL or
AC_CACHE_CHECK, because people are tempted to call
AC_DEFINE in the commands-to-set-it. Instead, the code that
follows the call to AC_CACHE_VAL should call
AC_DEFINE, by examining the value of the cache variable. For
instance, the following macro is broken:

Also, commands-to-set-it should not print any messages, for
example with AC_MSG_CHECKING; do that before calling
AC_CACHE_VAL, so the messages are printed regardless of whether
the results of the check are retrieved from the cache or determined by
running the shell commands.

7.4.1 Cache Variable Names

for example, ‘ac_cv_header_stat_broken’ or
‘ac_cv_prog_gcc_traditional’. The parts of the variable name are:

package-prefix

An abbreviation for your package or organization; the same prefix you
begin local Autoconf macros with, except lowercase by convention.
For cache values used by the distributed Autoconf macros, this value is
‘ac’.

_cv_

Indicates that this shell variable is a cache value. This string
must be present in the variable name, including the leading
underscore.

value-type

A convention for classifying cache values, to produce a rational naming
system. The values used in Autoconf are listed in Macro Names.

specific-value

Which member of the class of cache values this test applies to.
For example, which function (‘alloca’), program (‘gcc’), or
output variable (‘INSTALL’).

additional-options

Any particular behavior of the specific member that this test applies to.
For example, ‘broken’ or ‘set’. This part of the name may
be omitted if it does not apply.

The values assigned to cache variables may not contain newlines.
Usually, their values are Boolean (‘yes’ or ‘no’) or the
names of files or functions; so this is not an important restriction.
Cache Variable Index for an index of cache variables with
documented semantics.

7.4.2 Cache Files

A cache file is a shell script that caches the results of configure
tests run on one system so they can be shared between configure scripts
and configure runs. It is not useful on other systems. If its contents
are invalid for some reason, the user may delete or edit it, or override
documented cache variables on the configure command line.

By default, configure uses no cache file,
to avoid problems caused by accidental
use of stale cache files.

To enable caching, configure accepts --config-cache (or
-C) to cache results in the file config.cache.
Alternatively, --cache-file=file specifies that
file be the cache file. The cache file is created if it does not
exist already. When configure calls configure scripts in
subdirectories, it uses the --cache-file argument so that they
share the same cache. See Subdirectories, for information on
configuring subdirectories with the AC_CONFIG_SUBDIRS macro.

config.status only pays attention to the cache file if it is
given the --recheck option, which makes it rerun
configure.

It is wrong to try to distribute cache files for particular system types.
There is too much room for error in doing that, and too much
administrative overhead in maintaining them. For any features that
can't be guessed automatically, use the standard method of the canonical
system type and linking files (see Manual Configuration).

The site initialization script can specify a site-wide cache file to
use, instead of the usual per-program cache. In this case, the cache
file gradually accumulates information whenever someone runs a new
configure script. (Running configure merges the new cache
results with the existing cache file.) This may cause problems,
however, if the system configuration (e.g., the installed libraries or
compilers) changes and the stale cache file is not deleted.

If configure is interrupted at the right time when it updates
a cache file outside of the build directory where the configure
script is run, it may leave behind a temporary file named after the
cache file with digits following it. You may safely delete such a file.

7.4.3 Cache Checkpointing

If your configure script, or a macro called from configure.ac, happens
to abort the configure process, it may be useful to checkpoint the cache
a few times at key points using AC_CACHE_SAVE. Doing so
reduces the amount of time it takes to rerun the configure script with
(hopefully) the error that caused the previous abort corrected.

— Macro: AC_CACHE_LOAD

Loads values from existing cache file, or creates a new cache file if a
cache file is not found. Called automatically from AC_INIT.

— Macro: AC_CACHE_SAVE

Flushes all cached values to the cache file. Called automatically from
AC_OUTPUT, but it can be quite useful to call
AC_CACHE_SAVE at key points in configure.ac.

7.5 Printing Messages

configure scripts need to give users running them several kinds
of information. The following macros print messages in ways appropriate
for each kind. The arguments to all of them get enclosed in shell
double quotes, so the shell performs variable and back-quote
substitution on them.

These macros are all wrappers around the echo shell command.
They direct output to the appropriate file descriptor (see File Descriptor Macros).
configure scripts should rarely need to run echo directly
to print messages for the user. Using these macros makes it easy to
change how and when each kind of message is printed; such changes need
only be made to the macro definitions and all the callers change
automatically.

Notify the user that configure is checking for a particular
feature. This macro prints a message that starts with ‘checking ’
and ends with ‘...’ and no newline. It must be followed by a call
to AC_MSG_RESULT to print the result of the check and the
newline. The feature-description should be something like
‘whether the Fortran compiler accepts C++ comments’ or ‘for
c89’.

This macro prints nothing if configure is run with the
--quiet or --silent option.

— Macro: AC_MSG_RESULT (result-description)

Notify the user of the results of a check. result-description is
almost always the value of the cache variable for the check, typically
‘yes’, ‘no’, or a file name. This macro should follow a call
to AC_MSG_CHECKING, and the result-description should be
the completion of the message printed by the call to
AC_MSG_CHECKING.

This macro prints nothing if configure is run with the
--quiet or --silent option.

— Macro: AC_MSG_NOTICE (message)

Deliver the message to the user. It is useful mainly to print a
general description of the overall purpose of a group of feature checks,
e.g.,

AC_MSG_NOTICE([checking if stack overflow is detectable])

This macro prints nothing if configure is run with the
--quiet or --silent option.

— Macro: AC_MSG_ERROR (error-description, [exit-status = ‘$?/1’])

Notify the user of an error that prevents configure from
completing. This macro prints an error message to the standard error
output and exits configure with exit-status (‘$?’
by default, except that ‘0’ is converted to ‘1’).
error-description should be something like ‘invalid value
$HOME for \$HOME’.

The error-description should start with a lower-case letter, and
“cannot” is preferred to “can't”.

— Macro: AC_MSG_FAILURE (error-description, [exit-status])

This AC_MSG_ERROR wrapper notifies the user of an error that
prevents configure from completing and that additional
details are provided in config.log. This is typically used when
abnormal results are found during a compilation.

— Macro: AC_MSG_WARN (problem-description)

Notify the configure user of a possible problem. This macro
prints the message to the standard error output; configure
continues running afterward, so macros that call AC_MSG_WARN should
provide a default (back-up) behavior for the situations they warn about.
problem-description should be something like ‘ln -s seems to
make hard links’.

8.1 M4 Quotation

The most common problem with existing macros is an improper quotation.
This section, which users of Autoconf can skip, but which macro writers
must read, first justifies the quotation scheme that was chosen
for Autoconf and then ends with a rule of thumb. Understanding the
former helps one to follow the latter.

8.1.1 Active Characters

To fully understand where proper quotation is important, you first need
to know what the special characters are in Autoconf: ‘#’ introduces
a comment inside which no macro expansion is performed, ‘,’
separates arguments, ‘[’ and ‘]’ are the quotes
themselves3,
‘(’ and ‘)’ (which M4 tries to match by pairs), and finally
‘$’ inside a macro definition.

In order to understand the delicate case of macro calls, we first have
to present some obvious failures. Below they are “obvious-ified”,
but when you find them in real life, they are usually in disguise.

Comments, introduced by a hash and running up to the newline, are opaque
tokens to the top level: active characters are turned off, and there is
no macro expansion:

# define([def], ine)
⇒# define([def], ine)

Each time there can be a macro expansion, there is a quotation
expansion, i.e., one level of quotes is stripped:

int tab[10];
⇒int tab10;
[int tab[10];]
⇒int tab[10];

Without this in mind, the reader might try hopelessly to use her macro
array:

8.1.2 One Macro Call

Let's proceed on the interaction between active characters and macros
with this small macro, which just returns its first argument:

define([car], [$1])

The two pairs of quotes above are not part of the arguments of
define; rather, they are understood by the top level when it
tries to find the arguments of define. Therefore, assuming
car is not already defined, it is equivalent to write:

define(car, $1)

But, while it is acceptable for a configure.ac to avoid unnecessary
quotes, it is bad practice for Autoconf macros which must both be more
robust and also advocate perfect style.

At the top level, there are only two possibilities: either you
quote or you don't:

car(foo, bar, baz)
⇒foo
[car(foo, bar, baz)]
⇒car(foo, bar, baz)

Let's pay attention to the special characters:

car(#)
error-->EOF in argument list

The closing parenthesis is hidden in the comment; with a hypothetical
quoting, the top level understood it this way:

8.1.3 Quoting and Parameters

When M4 encounters ‘$’ within a macro definition, followed
immediately by a character it recognizes (‘0’...‘9’,
‘#’, ‘@’, or ‘*’), it will perform M4 parameter
expansion. This happens regardless of how many layers of quotes the
parameter expansion is nested within, or even if it occurs in text that
will be rescanned as a comment.

On the other hand, since autoconf generates shell code, you often want
to output shell variable expansion, rather than performing M4 parameter
expansion. To do this, you must use M4 quoting to separate the ‘$’
from the next character in the definition of your macro. If the macro
definition occurs in single-quoted text, then insert another level of
quoting; if the usage is already inside a double-quoted string, then
split it into concatenated strings.

Posix states that M4 implementations are free to provide implementation
extensions when ‘${’ is encountered in a macro definition.
Autoconf reserves the longer sequence ‘${{’ for use with planned
extensions that will be available in the future GNU M4 2.0,
but guarantees that all other instances of ‘${’ will be output
literally. Therefore, this idiom can also be used to output shell code
parameter references:

define([first], [${1}])first
⇒${1}

Posix also states that ‘$11’ should expand to the first parameter
concatenated with a literal ‘1’, although some versions of
GNU M4 expand the eleventh parameter instead. For
portability, you should only use single-digit M4 parameter expansion.

With this in mind, we can explore the cases where macros invoke
macros...

Huh? The first case is easily understood, but why is the second wrong,
and the third right? To understand that, you must know that after
M4 expands a macro, the resulting text is immediately subjected
to macro expansion and quote removal. This means that the quote removal
occurs twice—first before the argument is passed to the car
macro, and second after the car macro expands to the first
argument.

As the author of the Autoconf macro car, you then consider it to
be incorrect that your users have to double-quote the arguments of
car, so you “fix” your macro. Let's call it qar for
quoted car:

define([qar], [[$1]])

and check that qar is properly fixed:

qar([int tab[10];])
⇒int tab[10];

Ahhh! That's much better.

But note what you've done: now that the result of qar is always
a literal string, the only time a user can use nested macros is if she
relies on an unquoted macro call:

qar(active)
⇒ACT
qar([active])
⇒active

leaving no way for her to reproduce what she used to do with car:

car([active])
⇒ACT, IVE

Worse yet: she wants to use a macro that produces a set of cpp
macros:

This macro, qar, because it double quotes its arguments, forces
its users to leave their macro calls unquoted, which is dangerous.
Commas and other active symbols are interpreted by M4 before
they are given to the macro, often not in the way the users expect.
Also, because qar behaves differently from the other macros,
it's an exception that should be avoided in Autoconf.

8.1.5 changequote is Evil

The temptation is often high to bypass proper quotation, in particular
when it's late at night. Then, many experienced Autoconf hackers
finally surrender to the dark side of the force and use the ultimate
weapon: changequote.

The M4 builtin changequote belongs to a set of primitives that
allow one to adjust the syntax of the language to adjust it to one's
needs. For instance, by default M4 uses ‘`’ and ‘'’ as
quotes, but in the context of shell programming (and actually of most
programming languages), that's about the worst choice one can make:
because of strings and back-quoted expressions in shell code (such as
‘'this'’ and ‘`that`’), and because of literal characters in usual
programming languages (as in ‘'0'’), there are many unbalanced
‘`’ and ‘'’. Proper M4 quotation then becomes a nightmare, if
not impossible. In order to make M4 useful in such a context, its
designers have equipped it with changequote, which makes it
possible to choose another pair of quotes. M4sugar, M4sh, Autoconf, and
Autotest all have chosen to use ‘[’ and ‘]’. Not especially
because they are unlikely characters, but because they are
characters unlikely to be unbalanced.

There are other magic primitives, such as changecom to specify
what syntactic forms are comments (it is common to see
‘changecom(<!--, -->)’ when M4 is used to produce HTML pages),
changeword and changesyntax to change other syntactic
details (such as the character to denote the nth argument, ‘$’ by
default, the parentheses around arguments, etc.).

These primitives are really meant to make M4 more useful for specific
domains: they should be considered like command line options:
--quotes, --comments, --words, and
--syntax. Nevertheless, they are implemented as M4 builtins, as
it makes M4 libraries self contained (no need for additional options).

There lies the problem...

The problem is that it is then tempting to use them in the middle of an
M4 script, as opposed to its initialization. This, if not carefully
thought out, can lead to disastrous effects: you are changing the
language in the middle of the execution. Changing and restoring the
syntax is often not enough: if you happened to invoke macros in between,
these macros are lost, as the current syntax is probably not
the one they were implemented with.

8.1.6 Quadrigraphs

When writing an Autoconf macro you may occasionally need to generate
special characters that are difficult to express with the standard
Autoconf quoting rules. For example, you may need to output the regular
expression ‘[^[]’, which matches any character other than ‘[’.
This expression contains unbalanced brackets so it cannot be put easily
into an M4 macro.

Additionally, there are a few m4sugar macros (such as m4_split
and m4_expand) which internally use special markers in addition
to the regular quoting characters. If the arguments to these macros
contain the literal strings ‘-=<{(’ or ‘)}>=-’, the macros
might behave incorrectly.

You can work around these problems by using one of the following
quadrigraphs:

‘@<:@’

‘[’

‘@:>@’

‘]’

‘@S|@’

‘$’

‘@%:@’

‘#’

‘@{:@’

‘(’

‘@:}@’

‘)’

‘@&t@’

Expands to nothing.

Quadrigraphs are replaced at a late stage of the translation process,
after m4 is run, so they do not get in the way of M4 quoting.
For example, the string ‘^@<:@’, independently of its quotation,
appears as ‘^[’ in the output.

The empty quadrigraph can be used:

to mark trailing spaces explicitly

Trailing spaces are smashed by autom4te. This is a feature.

to produce quadrigraphs and other strings reserved by m4sugar

For instance ‘@<@&t@:@’ produces ‘@<:@’. For a more
contrived example:

For instance you might want to mention AC_FOO in a comment, while
still being sure that autom4te still catches unexpanded
‘AC_*’. Then write ‘AC@&t@_FOO’.

The name ‘@&t@’ was suggested by Paul Eggert:

I should give some credit to the ‘@&t@’ pun. The ‘&’ is my
own invention, but the ‘t’ came from the source code of the
ALGOL68C compiler, written by Steve Bourne (of Bourne shell fame),
and which used ‘mt’ to denote the empty string. In C, it would
have looked like something like:

char const mt[] = "";

but of course the source code was written in Algol 68.

I don't know where he got ‘mt’ from: it could have been his own
invention, and I suppose it could have been a common pun around the
Cambridge University computer lab at the time.

8.1.7 Dealing with unbalanced parentheses

One of the pitfalls of portable shell programming is that case
statements require unbalanced parentheses (see Limitations of Shell Builtins). With syntax highlighting
editors, the presence of unbalanced ‘)’ can interfere with editors
that perform syntax highlighting of macro contents based on finding the
matching ‘(’. Another concern is how much editing must be done
when transferring code snippets between shell scripts and macro
definitions. But most importantly, the presence of unbalanced
parentheses can introduce expansion bugs.

For an example, here is an underquoted attempt to use the macro
my_case, which happens to expand to a portable case
statement:

In the above example, the AS_IF call underquotes its arguments.
As a result, the unbalanced ‘)’ generated by the premature
expansion of my_case results in expanding AS_IF with a
truncated parameter, and the expansion is syntactically invalid:

if :; then
case $file_name in
*.c
fi echo "C source code";;
esac)

If nothing else, this should emphasize the importance of the quoting
arguments to macro calls. On the other hand, there are several
variations for defining my_case to be more robust, even when used
without proper quoting, each with some benefits and some drawbacks.

This version provides balanced parentheses to several editors, and can
be copied and pasted into a terminal as is. Unfortunately, it is still
unbalanced as an Autoconf argument, since ‘#(’ is an M4 comment
that masks the normal properties of ‘(’.

This version provides balanced parentheses to even more editors, and can
be used as a balanced Autoconf argument. Unfortunately, it requires
some editing before it can be copied and pasted into a terminal, and the
use of the quadrigraph ‘@%:@’ for ‘#’ reduces readability.

This version quotes the ‘)’, so that it can be used as a balanced
Autoconf argument. As written, this is not balanced to an editor, but
it can be coupled with ‘[#(]’ to meet that need, too. However, it
still requires some edits before it can be copied and pasted into a
terminal.

Since the entire macro is double-quoted, there is no problem with using
this as an Autoconf argument; and since the double-quoting is over the
entire statement, this code can be easily copied and pasted into a
terminal. However, the double quoting prevents the expansion of any
macros inside the case statement, which may cause its own set of
problems.

This version avoids the balancing issue altogether, by relying on
AS_CASE (see Common Shell Constructs); it also allows for the
expansion of AC_REQUIRE to occur prior to the entire case
statement, rather than within a branch of the case statement that might
not be taken. However, the abstraction comes with a penalty that it is
no longer a quick copy, paste, and edit to get back to shell code.

8.1.8 Quotation Rule Of Thumb

To conclude, the quotation rule of thumb is:

One pair of quotes per pair of parentheses.

Never over-quote, never under-quote, in particular in the definition of
macros. In the few places where the macros need to use brackets
(usually in C program text or regular expressions), properly quote
the arguments!

The M4-fluent reader might note that these two examples are rigorously
equivalent, since M4 swallows both the ‘changequote(<<, >>)’
and ‘<<’ ‘>>’ when it collects the arguments: these
quotes are not part of the arguments!

Simplified, the example above is just doing this:

changequote(<<, >>)dnl
<<[]>>
changequote([, ])dnl

instead of simply:

[[]]

With macros that do not double quote their arguments (which is the
rule), double-quote the (risky) literals:

Please note that the macro AC_TRY_LINK is obsolete, so you really
should be using AC_LINK_IFELSE instead.

See Quadrigraphs, for what to do if you run into a hopeless case
where quoting does not suffice.

When you create a configure script using newly written macros,
examine it carefully to check whether you need to add more quotes in
your macros. If one or more words have disappeared in the M4
output, you need more quotes. When in doubt, quote.

However, it's also possible to put on too many layers of quotes. If
this happens, the resulting configure script may contain
unexpanded macros. The autoconf program checks for this problem
by looking for the string ‘AC_’ in configure. However, this
heuristic does not work in general: for example, it does not catch
overquoting in AC_DEFINE descriptions.

8.2.1 Invoking autom4te

The command line arguments are modeled after M4's:

autom4te optionsfiles

where the files are directly passed to m4. By default,
GNU M4 is found during configuration, but the environment
variable
M4 can be set to tell autom4te where to look. In addition
to the regular expansion, it handles the replacement of the quadrigraphs
(see Quadrigraphs), and of ‘__oline__’, the current line in the
output. It supports an extended syntax for the files:

file.m4f

This file is an M4 frozen file. Note that all the previous files
are ignored. See the option --melt for the rationale.

file?

If found in the library path, the file is included for expansion,
otherwise it is ignored instead of triggering a failure.

Of course, it supports the Autoconf common subset of options:

--help

-h

Print a summary of the command line options and exit.

--version

-V

Print the version number of Autoconf and exit.

--verbose

-v

Report processing steps.

--debug

-d

Don't remove the temporary files and be even more verbose.

--include=dir

-I dir

Also look for input files in dir. Multiple invocations
accumulate.

--output=file

-o file

Save output (script or trace) to file. The file - stands
for the standard output.

As an extension of m4, it includes the following options:

--warnings=category

-W category

Report the warnings related to category (which can actually be a
comma separated list). See Reporting Messages, macro
AC_DIAGNOSE, for a comprehensive list of categories. Special
values include:

‘all’

report all the warnings

‘none’

report none

‘error’

treats warnings as errors

‘no-category’

disable warnings falling into category

Warnings about ‘syntax’ are enabled by default, and the environment
variable WARNINGS, a comma separated list of categories, is
honored. ‘autom4te -W category’ actually
behaves as if you had run:

autom4te --warnings=syntax,$WARNINGS,category

For example, if you want to disable defaults and WARNINGS
of autom4te, but enable the warnings about obsolete
constructs, you would use -W none,obsolete.

autom4te displays a back trace for errors, but not for
warnings; if you want them, just pass -W error.

--melt

-M

Do not use frozen files. Any argument file.m4f is
replaced by file.m4. This helps tracing the macros which
are executed only when the files are frozen, typically
m4_define. For instance, running:

autom4te --melt 1.m4 2.m4f 3.m4 4.m4f input.m4

is roughly equivalent to running:

m4 1.m4 2.m4 3.m4 4.m4 input.m4

while

autom4te 1.m4 2.m4f 3.m4 4.m4f input.m4

is equivalent to:

m4 --reload-state=4.m4f input.m4

--freeze

-F

Produce a frozen state file. autom4te freezing is stricter
than M4's: it must produce no warnings, and no output other than empty
lines (a line with white space is not empty) and comments
(starting with ‘#’). Unlike m4's similarly-named option,
this option takes no argument:

autom4te 1.m4 2.m4 3.m4 --freeze --output=3.m4f

corresponds to

m4 1.m4 2.m4 3.m4 --freeze-state=3.m4f

--mode=octal-mode

-m octal-mode

Set the mode of the non-traces output to octal-mode; by default
‘0666’.

As another additional feature over m4, autom4te
caches its results. GNU M4 is able to produce a regular
output and traces at the same time. Traces are heavily used in the
GNU Build System: autoheader uses them to build
config.h.in, autoreconf to determine what
GNU Build System components are used, automake to
“parse” configure.ac etc. To avoid recomputation,
traces are cached while performing regular expansion,
and conversely. This cache is (actually, the caches are) stored in
the directory autom4te.cache. It can safely be removed
at any moment (especially if for some reason autom4te
considers it trashed).

--cache=directory

-C directory

Specify the name of the directory where the result should be cached.
Passing an empty value disables caching. Be sure to pass a relative
file name, as for the time being, global caches are not supported.

--no-cache

Don't cache the results.

--force

-f

If a cache is used, consider it obsolete (but update it anyway).

Because traces are so important to the GNU Build System,
autom4te provides high level tracing features as compared to
M4, and helps exploiting the cache:

--trace=macro[:format]

-t macro[:format]

Trace the invocations of macro according to the format.
Multiple --trace arguments can be used to list several macros.
Multiple --trace arguments for a single macro are not
cumulative; instead, you should just make format as long as
needed.

The format is a regular string, with newlines if desired, and
several special escape codes. It defaults to ‘$f:$l:$n:$%’. It can
use the following special escapes:

‘$$’

The character ‘$’.

‘$f’

The file name from which macro is called.

‘$l’

The line number from which macro is called.

‘$d’

The depth of the macro call. This is an M4 technical detail that
you probably don't want to know about.

‘$n’

The name of the macro.

‘$num’

The numth argument of the call to macro.

‘$@’

‘$sep@’

‘${separator}@’

All the arguments passed to macro, separated by the character
sep or the string separator (‘,’ by default). Each
argument is quoted, i.e., enclosed in a pair of square brackets.

‘$*’

‘$sep*’

‘${separator}*’

As above, but the arguments are not quoted.

‘$%’

‘$sep%’

‘${separator}%’

As above, but the arguments are not quoted, all new line characters in
the arguments are smashed, and the default separator is ‘:’.

The escape ‘$%’ produces single-line trace outputs (unless you put
newlines in the ‘separator’), while ‘$@’ and ‘$*’ do
not.

Cache the traces of macro, but do not enable traces. This is
especially important to save CPU cycles in the future. For instance,
when invoked, autoconf preselects all the macros that
autoheader, automake, autoreconf, etc.,
trace, so that running m4 is not needed to trace them: the
cache suffices. This results in a huge speed-up.

Finally, autom4te introduces the concept of Autom4te
libraries. They consists in a powerful yet extremely simple feature:
sets of combined command line arguments:

8.2.2 Customizing autom4te

One can customize autom4te via ~/.autom4te.cfg (i.e.,
as found in the user home directory), and ./.autom4te.cfg (i.e.,
as found in the directory from which autom4te is run). The
order is first reading autom4te.cfg, then ~/.autom4te.cfg,
then ./.autom4te.cfg, and finally the command line arguments.

In these text files, comments are introduced with #, and empty
lines are ignored. Customization is performed on a per-language basis,
wrapped in between a ‘begin-language: "language"’,
‘end-language: "language"’ pair.

Customizing a language stands for appending options (see autom4te Invocation) to the current definition of the language. Options, and
more generally arguments, are introduced by ‘args:
arguments’. You may use the traditional shell syntax to quote the
arguments.

As an example, to disable Autoconf caches (autom4te.cache)
globally, include the following lines in ~/.autom4te.cfg:

8.3 Programming in M4sugar

M4 by itself provides only a small, but sufficient, set of all-purpose
macros. M4sugar introduces additional generic macros. Its name was
coined by Lars J. Aas: “Readability And Greater Understanding Stands 4
M4sugar”.

M4sugar reserves the macro namespace ‘^_m4_’ for internal use, and
the macro namespace ‘^m4_’ for M4sugar macros. You should not
define your own macros into these namespaces.

8.3.1 Redefined M4 Macros

With a few exceptions, all the M4 native macros are moved in the
‘m4_’ pseudo-namespace, e.g., M4sugar renames define as
m4_define etc.

The list of macros unchanged from M4, except for their name, is:

m4_builtin

m4_changecom

m4_changequote

m4_debugfile

m4_debugmode

m4_decr

m4_define

m4_divnum

m4_errprint

m4_esyscmd

m4_eval

m4_format

m4_ifdef

m4_incr

m4_index

m4_indir

m4_len

m4_pushdef

m4_shift

m4_substr

m4_syscmd

m4_sysval

m4_traceoff

m4_traceon

m4_translit

Some M4 macros are redefined, and are slightly incompatible with their
native equivalent.

— Macro: __file__
— Macro: __line__

All M4 macros starting with ‘__’ retain their original name: for
example, no m4__file__ is defined.

— Macro: __oline__

This is not technically a macro, but a feature of Autom4te. The
sequence __oline__ can be used similarly to the other m4sugar
location macros, but rather than expanding to the location of the input
file, it is translated to the line number where it appears in the output
file after all other M4 expansions.

— Macro: dnl

This macro kept its original name: no m4_dnl is defined.

— Macro: m4_bpatsubst (string, regexp, [replacement])

This macro corresponds to patsubst. The name m4_patsubst
is kept for future versions of M4sugar, once GNU M4 2.0 is
released and supports extended regular expression syntax.

— Macro: m4_bregexp (string, regexp, [replacement])

This macro corresponds to regexp. The name m4_regexp
is kept for future versions of M4sugar, once GNU M4 2.0 is
released and supports extended regular expression syntax.

These macros aren't directly builtins, but are closely related to
m4_pushdef and m4_defn. m4_copy and
m4_rename ensure that dest is undefined, while
m4_copy_force and m4_rename_force overwrite any existing
definition. All four macros then proceed to copy the entire pushdef
stack of definitions of source over to dest. m4_copy
and m4_copy_force preserve the source (including in the special
case where source is undefined), while m4_rename and
m4_rename_force undefine the original macro name (making it an
error to rename an undefined source).

Note that attempting to invoke a renamed macro might not work, since the
macro may have a dependence on helper macros accessed via composition of
‘$0’ but that were not also renamed; likewise, other macros may
have a hard-coded dependence on source and could break if
source has been deleted. On the other hand, it is always safe to
rename a macro to temporarily move it out of the way, then rename it
back later to restore original semantics.

— Macro: m4_defn (macro...)

This macro fails if macro is not defined, even when using older
versions of M4 that did not warn. See m4_undefine.
Unfortunately, in order to support these older versions of M4, there are
some situations involving unbalanced quotes where concatenating multiple
macros together will work in newer M4 but not in m4sugar; use
quadrigraphs to work around this.

— Macro: m4_divert (diversion)

M4sugar relies heavily on diversions, so rather than behaving as a
primitive, m4_divert behaves like:

m4_divert_pop()m4_divert_push([diversion])

See Diversion support, for more details about the use of the
diversion stack. In particular, this implies that diversion
should be a named diversion rather than a raw number. But be aware that
it is seldom necessary to explicitly change the diversion stack, and
that when done incorrectly, it can lead to syntactically invalid
scripts.

— Macro: m4_dumpdef (name...)
— Macro: m4_dumpdefs (name...)

m4_dumpdef is like the M4 builtin, except that this version
requires at least one argument, output always goes to standard error
rather than the current debug file, no sorting is done on multiple
arguments, and an error is issued if any
name is undefined. m4_dumpdefs is a convenience macro that
calls m4_dumpdef for all of the
m4_pushdef stack of definitions, starting with the current, and
silently does nothing if name is undefined.

Unfortunately, due to a limitation in M4 1.4.x, any macro defined as a
builtin is output as the empty string. This behavior is rectified by
using M4 1.6 or newer. However, this behavior difference means that
m4_dumpdef should only be used while developing m4sugar macros,
and never in the final published form of a macro.

— Macro: m4_esyscmd_s (command)

Like m4_esyscmd, this macro expands to the result of running
command in a shell. The difference is that any trailing newlines
are removed, so that the output behaves more like shell command
substitution.

This macro corresponds to ifelse. string-1 and
string-2 are compared literally, so usually one of the two
arguments is passed unquoted. See Conditional constructs, for more
conditional idioms.

— Macro: m4_include (file)
— Macro: m4_sinclude (file)

Like the M4 builtins, but warn against multiple inclusions of file.

— Macro: m4_mkstemp (template)
— Macro: m4_maketemp (template)

Posix requires maketemp to replace the trailing ‘X’
characters in template with the process id, without regards to the
existence of a file by that name, but this a security hole. When this
was pointed out to the Posix folks, they agreed to invent a new macro
mkstemp that always creates a uniquely named file, but not all
versions of GNU M4 support the new macro. In M4sugar,
m4_maketemp and m4_mkstemp are synonyms for each other,
and both have the secure semantics regardless of which macro the
underlying M4 provides.

— Macro: m4_popdef (macro...)

This macro fails if macro is not defined, even when using older
versions of M4 that did not warn. See m4_undefine.

— Macro: m4_undefine (macro...)

This macro fails if macro is not defined, even when using older
versions of M4 that did not warn. Use

m4_ifdef([macro], [m4_undefine([macro])])

if you are not sure whether macro is defined.

— Macro: m4_undivert (diversion...)

Unlike the M4 builtin, at least one diversion must be specified.
Also, since the M4sugar diversion stack prefers named
diversions, the use of m4_undivert to include files is risky.
See Diversion support, for more details about the use of the
diversion stack. But be aware that it is seldom necessary to explicitly
change the diversion stack, and that when done incorrectly, it can lead
to syntactically invalid scripts.

— Macro: m4_wrap (text)
— Macro: m4_wrap_lifo (text)

These macros correspond to m4wrap. Posix requires arguments of
multiple wrap calls to be reprocessed at EOF in the same order
as the original calls (first-in, first-out). GNU M4 versions
through 1.4.10, however, reprocess them in reverse order (last-in,
first-out). Both orders are useful, therefore, you can rely on
m4_wrap to provide FIFO semantics and m4_wrap_lifo for
LIFO semantics, regardless of the underlying GNU M4 version.

Unlike the GNU M4 builtin, these macros only recognize one
argument, and avoid token pasting between consecutive invocations. On
the other hand, nested calls to m4_wrap from within wrapped text
work just as in the builtin.

8.3.2 Diagnostic messages from M4sugar

When macros statically diagnose abnormal situations, benign or fatal,
they should report them using these macros. For issuing dynamic issues,
i.e., when configure is run, see Printing Messages.

— Macro: m4_assert (expression, [exit-status = ‘1’])

Assert that the arithmetic expression evaluates to non-zero.
Otherwise, issue a fatal error, and exit autom4te with
exit-status.

— Macro: m4_errprintn (message)

Similar to the builtin m4_errprint, except that a newline is
guaranteed after message.

— Macro: m4_fatal (message)

Report a severe error message prefixed with the current location,
and have autom4te die.

— Macro: m4_location

Useful as a prefix in a message line. Short for:

__file__:__line__

— Macro: m4_warn (category, message)

Report message as a warning (or as an error if requested by the
user) if warnings of the category are turned on. If the message
is emitted, it is prefixed with the current location, and followed by a
call trace of all macros defined via AC_DEFUN used to get to the
current expansion. You are encouraged to use standard categories, which
currently include:

‘all’

messages that don't fall into one of the following categories. Use of an
empty category is equivalent.

8.3.3 Diversion support

M4sugar makes heavy use of diversions under the hood, because it is
often the case that
text that must appear early in the output is not discovered until late
in the input. Additionally, some of the topological sorting algorithms
used in resolving macro dependencies use diversions. However, most
macros should not need to change diversions directly, but rather rely on
higher-level M4sugar macros to manage diversions transparently. If you
change diversions improperly, you risk generating a syntactically
invalid script, because an incorrect diversion will violate assumptions
made by many macros about whether prerequisite text has been previously
output. In short, if you manually change the diversion, you should not
expect any macros provided by the Autoconf package to work until you
have restored the diversion stack back to its original state.

In the rare case that it is necessary to write a macro that explicitly
outputs text to a different diversion, it is important to be aware of an
M4 limitation regarding diversions: text only goes to a diversion if it
is not part of argument collection. Therefore, any macro that changes
the current diversion cannot be used as an unquoted argument to another
macro, but must be expanded at the top level. The macro
m4_expand will diagnose any attempt to change diversions, since
it is generally useful only as an argument to another macro. The
following example shows what happens when diversion manipulation is
attempted within macro arguments:

Notice that the unquoted text unwanted is output, even though it
was processed while the current diversion was KILL, because it
was collected as part of the argument to m4_do. However, the
text discarded disappeared as desired, because the diversion
changes were single-quoted, and were not expanded until the top-level
rescan of the output of m4_do.

To make diversion management easier, M4sugar uses the concept of named
diversions. Rather than using diversion numbers directly, it is nicer
to associate a name with each diversion. The diversion number associated
with a particular diversion name is an implementation detail, and a
syntax warning is issued if a diversion number is used instead of a
name. In general, you should not output text
to a named diversion until after calling the appropriate initialization
routine for your language (m4_init, AS_INIT,
AT_INIT, ...), although there are some exceptions documented
below.

M4sugar defines two named diversions.

KILL

Text written to this diversion is discarded. This is the default
diversion once M4sugar is initialized.

GROW

This diversion is used behind the scenes by topological sorting macros,
such as AC_REQUIRE.

M4sh adds several more named diversions.

BINSH

This diversion is reserved for the ‘#!’ interpreter line.

HEADER-REVISION

This diversion holds text from AC_REVISION.

HEADER-COMMENT

This diversion holds comments about the purpose of a file.

HEADER-COPYRIGHT

This diversion is managed by AC_COPYRIGHT.

M4SH-SANITIZE

This diversion contains M4sh sanitization code, used to ensure M4sh is
executing in a reasonable shell environment.

M4SH-INIT

This diversion contains M4sh initialization code, initializing variables
that are required by other M4sh macros.

BODY

This diversion contains the body of the shell code, and is the default
diversion once M4sh is initialized.

Autotest inherits diversions from M4sh, and changes the default
diversion from BODY back to KILL. It also adds several
more named diversions, with the following subset designed for developer
use.

PREPARE_TESTS

This diversion contains initialization sequences which are executed
after atconfig and atlocal, and after all command line
arguments have been parsed, but prior to running any tests. It can be
used to set up state that is required across all tests. This diversion
will work even before AT_INIT.

Autoconf inherits diversions from M4sh, and adds the following named
diversions which developers can utilize.

DEFAULTS

This diversion contains shell variable assignments to set defaults that
must be in place before arguments are parsed. This diversion is placed
early enough in configure that it is unsafe to expand any
autoconf macros into this diversion.

HELP_ENABLE

If AC_PRESERVE_HELP_ORDER was used, then text placed in this
diversion will be included as part of a quoted here-doc providing all of
the --help output of configure related to options
created by AC_ARG_WITH and AC_ARG_ENABLE.

INIT_PREPARE

This diversion occurs after all command line options have been parsed,
but prior to the main body of the configure script. This
diversion is the last chance to insert shell code such as variable
assignments or shell function declarations that will used by the
expansion of other macros.

For now, the remaining named diversions of Autoconf, Autoheader, and
Autotest are not documented. In other words,
intentionally outputting text into an undocumented diversion is subject
to breakage in a future release of Autoconf.

— Macro: m4_cleardivert (diversion...)

Permanently discard any text that has been diverted into
diversion.

— Macro: m4_divert_once (diversion, [content])

Similar to m4_divert_text, except that content is only
output to diversion if this is the first time that
m4_divert_once has been called with its particular arguments.

— Macro: m4_divert_pop ([diversion])

If provided, check that the current diversion is indeed diversion.
Then change to the diversion located earlier on the stack, giving an
error if an attempt is made to pop beyond the initial m4sugar diversion
of KILL.

— Macro: m4_divert_push (diversion)

Remember the former diversion on the diversion stack, and output
subsequent text into diversion. M4sugar maintains a diversion
stack, and issues an error if there is not a matching pop for every
push.

— Macro: m4_divert_text (diversion, [content])

Output content and a newline into diversion, without
affecting the current diversion. Shorthand for:

m4_divert_push([diversion])content
m4_divert_pop([diversion])dnl

One use of m4_divert_text is to develop two related macros, where
macro ‘MY_A’ does the work, but adjusts what work is performed
based on whether the optional macro ‘MY_B’ has also been expanded.
Of course, it is possible to use AC_BEFORE within MY_A to
require that ‘MY_B’ occurs first, if it occurs at all. But this
imposes an ordering restriction on the user; it would be nicer if macros
‘MY_A’ and ‘MY_B’ can be invoked in either order. The trick
is to let ‘MY_B’ leave a breadcrumb in an early diversion, which
‘MY_A’ can then use to determine whether ‘MY_B’ has been
expanded.

8.3.4 Conditional constructs

The string string is repeatedly compared against a series of
regex arguments; if a match is found, the expansion is the
corresponding value, otherwise, the macro moves on to the next
regex. If no regex match, then the result is the optional
default, or nothing.

The result of the substitution is then passed through the next set of
regex and subst, and so forth. An empty subst implies
deletion of any matched portions in the current string. Note that this
macro over-quotes string; this behavior is intentional, so that
the result of each step of the recursion remains as a quoted string.
However, it means that anchors (‘^’ and ‘$’ in the regex
will line up with the extra quotations, and not the characters of the
original string. The overquoting is removed after the final
substitution.

This macro was introduced in Autoconf 2.62. Similar to m4_if,
except that each test is expanded only when it is encountered.
This is useful for short-circuiting expensive tests; while m4_if
requires all its strings to be expanded up front before doing
comparisons, m4_cond only expands a test when all earlier
tests have failed.

For an example, these two sequences give the same result, but in the
case where ‘$1’ does not contain a backslash, the m4_cond
version only expands m4_index once, instead of five times, for
faster computation if this is a common case for ‘$1’. Notice that
every third argument is unquoted for m4_if, and quoted for
m4_cond:

If expr-1 contains text, use it. Otherwise, select expr-2.
m4_default expands the result, while m4_default_quoted
does not. Useful for providing a fixed default if the expression that
results in expr-1 would otherwise be empty. The difference
between m4_default and m4_default_nblank is whether an
argument consisting of just blanks (space, tab, newline) is
significant. When using the expanding versions, note that an argument
may contain text but still expand to an empty string.

If cond is empty or consists only of blanks (space, tab, newline),
then expand if-blank; otherwise, expand if-text. Two
variants exist, in order to make it easier to select the correct logical
sense when using only two parameters. Note that this is more efficient
than the equivalent behavior of:

m4_ifval(m4_normalize([cond]), if-text, if-blank)

— Macro: m4_ifndef (macro, if-not-defined, [if-defined])

This is shorthand for:

m4_ifdef([macro], [if-defined], [if-not-defined])

— Macro: m4_ifset (macro, [if-true], [if-false])

If macro is undefined, or is defined as the empty string, expand
to if-false. Otherwise, expands to if-true. Similar to:

m4_ifval(m4_defn([macro]), [if-true], [if-false])

except that it is not an error if macro is undefined.

— Macro: m4_ifval (cond, [if-true], [if-false])

Expands to if-true if cond is not empty, otherwise to
if-false. This is shorthand for:

m4_if([cond], [], [if-false], [if-true])

— Macro: m4_ifvaln (cond, [if-true], [if-false])

Similar to m4_ifval, except guarantee that a newline is present
after any non-empty expansion. Often followed by dnl.

— Macro: m4_n (text)

Expand to text, and add a newline if text is not empty.
Often followed by dnl.

8.3.5 Looping constructs

The following macros are useful in implementing recursive algorithms in
M4, including loop operations. An M4 list is formed by quoting a list
of quoted elements; generally the lists are comma-separated, although
m4_foreach_w is whitespace-separated. For example, the list
‘[[a], [b,c]]’ contains two elements: ‘[a]’ and ‘[b,c]’.
It is common to see lists with unquoted elements when those elements are
not likely to be macro names, as in ‘[fputc_unlocked,
fgetc_unlocked]’.

Although not generally recommended, it is possible for quoted lists to
have side effects; all side effects are expanded only once, and prior to
visiting any list element. On the other hand, the fact that unquoted
macros are expanded exactly once means that macros without side effects
can be used to generate lists. For example,

Extracts argument n (larger than 0) from the remaining arguments.
If there are too few arguments, the empty string is used. For any
n besides 1, this is more efficient than the similar
‘m4_car(m4_shiftn([n], [], [arg...]))’.

— Macro: m4_car (arg...)

Expands to the quoted first arg. Can be used with m4_cdr
to recursively iterate
through a list. Generally, when using quoted lists of quoted elements,
m4_car should be called without any extra quotes.

— Macro: m4_cdr (arg...)

Expands to a quoted list of all but the first arg, or the empty
string if there was only one argument. Generally, when using quoted
lists of quoted elements, m4_cdr should be called without any
extra quotes.

For example, this is a simple implementation of m4_map; note how
each iteration checks for the end of recursion, then merely applies the
first argument to the first element of the list, then repeats with the
rest of the list. (The actual implementation in M4sugar is a bit more
involved, to gain some speed and share code with m4_map_sep, and
also to avoid expanding side effects in ‘$2’ twice).

Loop over the numeric values between first and last
including bounds by increments of step. For each iteration,
expand expression with the numeric value assigned to var.
If step is omitted, it defaults to ‘1’ or ‘-1’ depending
on the order of the limits. If given, step has to match this
order. The number of iterations is determined independently from
definition of var; iteration cannot be short-circuited or
lengthened by modifying var from within expression.

— Macro: m4_foreach (var, list, expression)

Loop over the comma-separated M4 list list, assigning each value
to var, and expand expression. The following example
outputs two lines:

Loop over the comma separated quoted list of argument descriptions in
list, and invoke macro with the arguments. An argument
description is in turn a comma-separated quoted list of quoted elements,
suitable for m4_apply. The macros m4_map and
m4_map_sep ignore empty argument descriptions, while
m4_mapall and m4_mapall_sep invoke macro with no
arguments. The macros m4_map_sep and m4_mapall_sep
additionally expand separator between invocations of macro.

Note that separator is expanded, unlike in m4_join. When
separating output with commas, this means that the map result can be
used as a series of arguments, by using a single-quoted comma as
separator, or as a single string, by using a double-quoted comma.

Repeatedly invoke macro with each successive arg as its only
argument. In the following example, three solutions are presented with
the same expansion; the solution using m4_map_args is the most
efficient.

In cases where it is useful to operate on additional parameters besides
the list elements, the macro m4_curry can be used in macro
to supply the argument currying necessary to generate the desired
argument list. In the following example, list_add_n is more
efficient than list_add_x. On the other hand, using
m4_map_args_sep can be even more efficient.

Expand the sequence pre[arg]post for each
argument, additionally expanding sep between arguments. One
common use of this macro is constructing a macro call, where the opening
and closing parentheses are split between pre and post; in
particular, m4_map_args([macro], [arg]) is equivalent
to m4_map_args_sep([macro(], [)], [], [arg]). This
macro provides the most efficient means for iterating over an arbitrary
list of arguments, particularly when repeatedly constructing a macro
call with more arguments than arg.

— Macro: m4_map_args_w (string, [pre], [post], [sep])

Expand the sequence pre[word]post for each word in
the whitespace-separated string, additionally expanding sep
between words. This macro provides the most efficient means for
iterating over a whitespace-separated string. In particular,
m4_map_args_w([string], [action(], [)]) is more
efficient than m4_foreach_w([var], [string],
[action(m4_defn([var]))]).

m4_shiftn performs count iterations of m4_shift,
along with validation that enough arguments were passed in to match the
shift count, and that the count is positive. m4_shift2 and
m4_shift3 are specializations
of m4_shiftn, introduced in Autoconf 2.62, and are more efficient
for two and three shifts, respectively.

For each of the m4_pushdef definitions of macro, expand
action with the single argument of a definition of macro.
m4_stack_foreach starts with the oldest definition, while
m4_stack_foreach_lifo starts with the current definition.
action should not push or pop definitions of macro, nor is
there any guarantee that the current definition of macro matches
the argument that was passed to action. The macro m4_curry
can be used if action needs more than one argument, although in
that case it is more efficient to use m4_stack_foreach_sep.

Due to technical limitations, there are a few low-level m4sugar
functions, such as m4_pushdef, that cannot be used as the
macro argument.

Expand the sequence pre[definition]post for each
m4_pushdef definition of macro, additionally expanding
sep between definitions. m4_stack_foreach_sep visits the
oldest definition first, while m4_stack_foreach_sep_lifo visits
the current definition first. This macro provides the most efficient
means for iterating over a pushdef stack. In particular,
m4_stack_foreach([macro], [action]) is short for
m4_stack_foreach_sep([macro], [action(], [)]).

8.3.6 Evaluation Macros

The following macros give some control over the order of the evaluation
by adding or removing levels of quotes.

— Macro: m4_apply (macro, list)

Apply the elements of the quoted, comma-separated list as the
arguments to macro. If list is empty, invoke macro
without arguments. Note the difference between m4_indir, which
expects its first argument to be a macro name but can use names that are
otherwise invalid, and m4_apply, where macro can contain
other text, but must end in a valid macro name.

This macro returns the decimal count of the number of arguments it was
passed.

— Macro: m4_curry (macro, arg...)

This macro performs argument currying. The expansion of this macro is
another macro name that expects exactly one argument; that argument is
then appended to the arg list, and then macro is expanded
with the resulting argument list.

m4_curry([m4_curry], [m4_reverse], [1])([2])([3])
⇒3, 2, 1

Unfortunately, due to a limitation in M4 1.4.x, it is not possible to
pass the definition of a builtin macro as the argument to the output of
m4_curry; the empty string is used instead of the builtin token.
This behavior is rectified by using M4 1.6 or newer.

— Macro: m4_do (arg, ...)

This macro loops over its arguments and expands each arg in
sequence. Its main use is for readability; it allows the use of
indentation and fewer dnl to result in the same expansion. This
macro guarantees that no expansion will be concatenated with subsequent
text; to achieve full concatenation, use m4_unquote(m4_join([],
arg...)).

Return the arguments as a quoted list of quoted arguments.
Conveniently, if there is just one arg, this effectively adds a
level of quoting.

— Macro: m4_dquote_elt (arg, ...)

Return the arguments as a series of double-quoted arguments. Whereas
m4_dquote returns a single argument, m4_dquote_elt returns
as many arguments as it was passed.

— Macro: m4_echo (arg, ...)

Return the arguments, with the same level of quoting. Other than
discarding whitespace after unquoted commas, this macro is a no-op.

— Macro: m4_expand (arg)

Return the expansion of arg as a quoted string. Whereas
m4_quote is designed to collect expanded text into a single
argument, m4_expand is designed to perform one level of expansion
on quoted text. One distinction is in the treatment of whitespace
following a comma in the original arg. Any time multiple
arguments are collected into one with m4_quote, the M4 argument
collection rules discard the whitespace. However, with m4_expand,
whitespace is preserved, even after the expansion of macros contained in
arg. Additionally, m4_expand is able to expand text that
would involve an unterminated comment, whereas expanding that same text
as the argument to m4_quote runs into difficulty in finding the
end of the argument. Since manipulating diversions during argument
collection is inherently unsafe, m4_expand issues an error if
arg attempts to change the current diversion (see Diversion support).

Note that m4_expand cannot handle an arg that expands to
literal unbalanced quotes, but that quadrigraphs can be used when
unbalanced output is necessary. Likewise, unbalanced parentheses should
be supplied with double quoting or a quadrigraph.

This macro was introduced in Autoconf 2.62. Expands to nothing,
ignoring all of its arguments. By itself, this isn't very useful.
However, it can be used to conditionally ignore an arbitrary number of
arguments, by deciding which macro name to apply to a list of arguments.

dnl foo outputs a message only if [debug] is defined.
m4_define([foo],
[m4_ifdef([debug],[AC_MSG_NOTICE],[m4_ignore])([debug message])])

Note that for earlier versions of Autoconf, the macro __gnu__ can
serve the same purpose, although it is less readable.

— Macro: m4_make_list (arg, ...)

This macro exists to aid debugging of M4sugar algorithms. Its net
effect is similar to m4_dquote—it produces a quoted list of
quoted arguments, for each arg. The difference is that this
version uses a comma-newline separator instead of just comma, to improve
readability of the list; with the result that it is less efficient than
m4_dquote.

Return the arguments as a single entity, i.e., wrap them into a pair of
quotes. This effectively collapses multiple arguments into one,
although it loses whitespace after unquoted commas in the process.

— Macro: m4_reverse (arg, ...)

Outputs each argument with the same level of quoting, but in reverse
order, and with space following each comma for readability.

This macro was introduced in Autoconf 2.62. Expand each argument,
separated by commas. For a single arg, this effectively removes a
layer of quoting, and m4_unquote([arg]) is more efficient
than the equivalent m4_do([arg]). For multiple arguments,
this results in an unquoted list of expansions. This is commonly used
with m4_split, in order to convert a single quoted list into a
series of quoted elements.

The following example aims at emphasizing the difference between several
scenarios: not using these macros, using m4_defn, using
m4_quote, using m4_dquote, and using m4_expand.

8.3.7 String manipulation in M4

The following macros may be used to manipulate strings in M4. Many of
the macros in this section intentionally result in quoted strings as
output, rather than subjecting the arguments to further expansions. As
a result, if you are manipulating text that contains active M4
characters, the arguments are passed with single quoting rather than
double.

Redefine macro-name to its former contents with separator
and string added at the end. If macro-name was undefined
before (but not if it was defined but empty), then no separator is
added. As of Autoconf 2.62, neither string nor separator
are expanded during this macro; instead, they are expanded when
macro-name is invoked.

m4_append can be used to grow strings, and m4_append_uniq
to grow strings without duplicating substrings. Additionally,
m4_append_uniq takes two optional parameters as of Autoconf 2.62;
if-uniq is expanded if string was appended, and
if-duplicate is expanded if string was already present.
Also, m4_append_uniq warns if separator is not empty, but
occurs within string, since that can lead to duplicates.

Note that m4_append can scale linearly in the length of the final
string, depending on the quality of the underlying M4 implementation,
while m4_append_uniq has an inherent quadratic scaling factor.
If an algorithm can tolerate duplicates in the final string, use the
former for speed. If duplicates must be avoided, consider using
m4_set_add instead (see Set manipulation Macros).

This macro was introduced in Autoconf 2.62. It is similar to
m4_append_uniq, but treats strings as a whitespace
separated list of words to append, and only appends unique words.
macro-name is updated with a single space between new words.

Output string in quotes, but without a trailing newline. The
macro m4_chomp is slightly faster, and removes at most one
newline; the macro m4_chomp_all removes all consecutive trailing
newlines. Unlike m4_flatten, embedded newlines are left intact,
and backslash does not influence the result.

This macro produces a quoted string containing the pairwise combination
of every element of the quoted, comma-separated prefix-list, and
every element from the suffix arguments. Each pairwise
combination is joined with infix in the middle, and successive
pairs are joined by separator. No expansion occurs on any of the
arguments. No output occurs if either the prefix or suffix
list is empty, but the lists can contain empty elements.

Note that if all you intend to do is join args with commas between
them, to form a quoted list suitable for m4_foreach, it is more
efficient to use m4_dquote.

— Macro: m4_newline ([text])

This macro was introduced in Autoconf 2.62, and expands to a newline,
followed by any text.
It is primarily useful for maintaining macro formatting, and ensuring
that M4 does not discard leading whitespace during argument collection.

— Macro: m4_normalize (string)

Remove leading and trailing spaces and tabs, sequences of
backslash-then-newline, and replace multiple spaces, tabs, and newlines
with a single space. This is a combination of m4_flatten and
m4_strip. To determine if string consists only of bytes
that would be removed by m4_normalize, you can use
m4_ifblank.

— Macro: m4_re_escape (string)

Backslash-escape all characters in string that are active in
regexps.

— Macro: m4_split (string, [regexp = ‘[\t ]+’])

Split string into an M4 list of elements quoted by ‘[’ and
‘]’, while keeping white space at the beginning and at the end.
If regexp is given, use it instead of ‘[\t ]+’ for splitting.
If string is empty, the result is an empty list.

— Macro: m4_strip (string)

Strip whitespace from string. Sequences of spaces and tabs are
reduced to a single space, then leading and trailing spaces are removed.
The result is still a quoted string. Note that this does not interfere
with newlines; if you want newlines stripped as well, consider
m4_flatten, or do it all at once with m4_normalize. To
quickly test if string has only whitespace, use m4_ifblank.

— Macro: m4_text_box (message, [frame = ‘-’])

Add a text box around message, using frame as the border
character above and below the message. The frame argument must be
a single byte, and does not support quadrigraphs.
The frame correctly accounts for
the subsequent expansion of message. For example:

Break string into a series of whitespace-separated words, then
output those words separated by spaces, and wrapping lines any time the
output would exceed width columns. If given, prefix1 begins
the first line, and prefix begins all wrapped lines. If
prefix1 is longer than prefix, then the first line consists
of just prefix1. If prefix is longer than prefix1,
padding is inserted so that the first word of string begins at the
same indentation as all wrapped lines. Note that using literal tab
characters in any of the arguments will interfere with the calculation
of width. No expansions occur on prefix, prefix1, or the
words of string, although quadrigraphs are recognized.

8.3.8 Arithmetic computation in M4

The following macros facilitate integer arithmetic operations.
Where a parameter is documented as taking an arithmetic expression, you
can use anything that can be parsed by m4_eval.

— Macro: m4_cmp (expr-1, expr-2)

Compare the arithmetic expressions expr-1 and expr-2, and
expand to ‘-1’ if expr-1 is smaller, ‘0’ if they are
equal, and ‘1’ if expr-1 is larger.

— Macro: m4_list_cmp (list-1, list-2)

Compare the two M4 lists consisting of comma-separated arithmetic
expressions, left to right. Expand to ‘-1’ for the first element
pairing where the value from list-1 is smaller, ‘1’ where the
value from list-2 is smaller, or ‘0’ if both lists have the
same values. If one list is shorter than the other, the remaining
elements of the longer list are compared against zero.

This macro was introduced in Autoconf 2.62. Expand to the decimal value
of the maximum arithmetic expression among all the arguments.

— Macro: m4_min (arg, ...)

This macro was introduced in Autoconf 2.62. Expand to the decimal value
of the minimum arithmetic expression among all the arguments.

— Macro: m4_sign (expr)

Expand to ‘-1’ if the arithmetic expression expr is negative,
‘1’ if it is positive, and ‘0’ if it is zero.

— Macro: m4_version_compare (version-1, version-2)

This macro was introduced in Autoconf 2.53, but had a number of
usability limitations that were not lifted until Autoconf 2.62. Compare
the version strings version-1 and version-2, and expand to
‘-1’ if version-1 is smaller, ‘0’ if they are the same,
or ‘1’ version-2 is smaller. Version strings must be a list
of elements separated by ‘.’, ‘,’ or ‘-’, where each
element is a number along with optional case-insensitive letters
designating beta releases. The comparison stops at the leftmost element
that contains a difference, although a 0 element compares equal to a
missing element.

It is permissible to include commit identifiers in version, such
as an abbreviated SHA1 of the commit, provided there is still a
monotonically increasing prefix to allow for accurate version-based
comparisons. For example, this paragraph was written when the
development snapshot of autoconf claimed to be at version
‘2.61a-248-dc51’, or 248 commits after the 2.61a release, with an
abbreviated commit identification of ‘dc51’.

Compares version against the version of Autoconf currently
running. If the running version is at version or newer, expand
if-new-enough, but if version is larger than the version
currently executing, expand if-old, which defaults to printing an
error message and exiting m4sugar with status 63. When given only one
argument, this behaves like AC_PREREQ (see Versioning).
Remember that the autoconf philosophy favors feature checks over version
checks.

8.3.9 Set manipulation in M4

Sometimes, it is necessary to track a set of data, where the order does
not matter and where there are no duplicates in the set. The following
macros facilitate set manipulations. Each set is an opaque object,
which can only be accessed via these basic operations. The underlying
implementation guarantees linear scaling for set creation, which is more
efficient than using the quadratic m4_append_uniq. Both set
names and values can be arbitrary strings, except for unbalanced quotes.
This implementation ties up memory for removed elements until the next
operation that must traverse all the elements of a set; and although
that may slow down some operations until the memory for removed elements
is pruned, it still guarantees linear performance.

— Macro: m4_set_add (set, value, [if-uniq], [if-dup])

Adds the string value as a member of set set. Expand
if-uniq if the element was added, or if-dup if it was
previously in the set. Operates in amortized constant time, so that set
creation scales linearly.

— Macro: m4_set_add_all (set, value...)

Adds each value to the set set. This is slightly more
efficient than repeatedly invoking m4_set_add.

— Macro: m4_set_contains (set, value, [if-present], [if-absent])

Expands if-present if the string value is a member of
set, otherwise if-absent.

Expands to a single string consisting of all the members of the set
set, each separated by sep, which is not expanded.
m4_set_contents leaves the elements in set but reclaims any
memory occupied by removed elements, while m4_set_dump is a
faster one-shot action that also deletes the set. No provision is made
for disambiguating members that contain a non-empty sep as a
substring; use m4_set_empty to distinguish between an empty set
and the set containing only the empty string. The order of the output
is unspecified; in the current implementation, part of the speed of
m4_set_dump results from using a different output order than
m4_set_contents. These macros scale linearly in the size of the
set before memory pruning, and m4_set_contents([set],
[sep]) is faster than
m4_joinall([sep]m4_set_listc([set])).

Compute the relation between seta and setb, and output the
result as a list of quoted arguments without duplicates and with a
leading comma. Set difference selects the elements in seta but
not setb, intersection selects only elements in both sets, and
union selects elements in either set. These actions are linear in the
sum of the set sizes. The leading comma is necessary to distinguish
between no elements and the empty string as the only element.

Expand if-empty if the set set has no elements, otherwise
expand if-elements. This macro operates in constant time. Using
this macro can help disambiguate output from m4_set_contents or
m4_set_list.

— Macro: m4_set_foreach (set, variable, action)

For each element in the set set, expand action with the
macro variable defined as the set element. Behavior is
unspecified if action recursively lists the contents of set
(although listing other sets is acceptable), or if it modifies the set
in any way other than removing the element currently contained in
variable. This macro is faster than the corresponding
m4_foreach([variable],
m4_indir([m4_dquote]m4_set_listc([set])), [action]),
although m4_set_map might be faster still.

Produce a list of arguments, where each argument is a quoted element
from the set set. The variant m4_set_listc is unambiguous,
by adding a leading comma if there are any set elements, whereas the
variant m4_set_list cannot distinguish between an empty set and a
set containing only the empty string. These can be directly used in
macros that take multiple arguments, such as m4_join or
m4_set_add_all, or wrapped by m4_dquote for macros that
take a quoted list, such as m4_map or m4_foreach. Any
memory occupied by removed elements is reclaimed during these macros.

For each element in the set set, expand action with a single
argument of the set element. Behavior is unspecified if action
recursively lists the contents of set (although listing other sets
is acceptable), or if it modifies the set in any way other than removing
the element passed as an argument. This macro is faster than either
corresponding counterpart of
m4_map_args([action]m4_set_listc([set])) or
m4_set_foreach([set], [var],
[action(m4_defn([var]))]). It is possible to use m4_curry
if more than one argument is needed for action, although it is
more efficient to use m4_set_map_sep in that case.

— Macro: m4_set_map_sep (set, [pre], [post], [sep])

For each element in the set set, expand
pre[element]post, additionally expanding sep
between elements. Behavior is unspecified if the expansion recursively
lists the contents of set (although listing other sets
is acceptable), or if it modifies the set in any way other than removing
the element visited by the expansion. This macro provides the most
efficient means for non-destructively visiting the elements of a set; in
particular, m4_set_map([set], [action]) is equivalent
to m4_set_map_sep([set], [action(], [)]).

— Macro: m4_set_remove (set, value, [if-present], [if-absent])

If value is an element in the set set, then remove it and
expand if-present. Otherwise expand if-absent. This macro
operates in constant time so that multiple removals will scale linearly
rather than quadratically; but when used outside of
m4_set_foreach or m4_set_map, it leaves memory occupied
until the set is later
compacted by m4_set_contents or m4_set_list. Several
other set operations are then less efficient between the time of element
removal and subsequent memory compaction, but still maintain their
guaranteed scaling performance.

— Macro: m4_set_size (set)

Expand to the size of the set set. This implementation operates
in constant time, and is thus more efficient than
m4_eval(m4_count(m4_set_listc([set])) - 1).

8.3.10 Forbidden Patterns

M4sugar provides a means to define suspicious patterns, patterns
describing tokens which should not be found in the output. For
instance, if an Autoconf configure script includes tokens such as
‘AC_DEFINE’, or ‘dnl’, then most probably something went
wrong (typically a macro was not evaluated because of overquotation).

M4sugar forbids all the tokens matching ‘^_?m4_’ and ‘^dnl$’.
Additional layers, such as M4sh and Autoconf, add additional forbidden
patterns to the list.

— Macro: m4_pattern_forbid (pattern)

Declare that no token matching pattern must be found in the output.
Comments are not checked; this can be a problem if, for instance, you
have some macro left unexpanded after an ‘#include’. No consensus
is currently found in the Autoconf community, as some people consider it
should be valid to name macros in comments (which doesn't make sense to
the authors of this documentation: input, such as macros, should be
documented by ‘dnl’ comments; reserving ‘#’-comments to
document the output).

Of course, you might encounter exceptions to these generic rules, for
instance you might have to refer to ‘$m4_flags’.

— Macro: m4_pattern_allow (pattern)

Any token matching pattern is allowed, including if it matches an
m4_pattern_forbid pattern.

8.4 Debugging via autom4te

At times, it is desirable to see what was happening inside m4, to see
why output was not matching expectations. However, post-processing done
by autom4te means that directly using the m4 builtin
m4_traceon is likely to interfere with operation. Also, frequent
diversion changes and the concept of forbidden tokens make it difficult
to use m4_defn to generate inline comments in the final output.

There are a couple of tools to help with this. One is the use of the
--trace option provided by autom4te (as well as each
of the programs that wrap autom4te, such as
autoconf), in order to inspect when a macro is called and with
which arguments. For example, when this paragraph was written, the
autoconf version could be found by:

Another trick is to print out the expansion of various m4 expressions to
standard error or to an independent file, with no further m4 expansion,
and without interfering with diversion changes or the post-processing
done to standard output. m4_errprintn shows a given expression
on standard error. For example, if you want to see the expansion of an
autoconf primitive or of one of your autoconf macros, you can do it like
this:

A mass of mixed ingredients reduced to a soft pulpy state by beating or
pressure...

A mixture of meal or bran and water fed to animals.

A mess; trouble. [Obs.] –Beau. & Fl.

M4sh reserves the M4 macro namespace ‘^_AS_’ for internal use, and
the namespace ‘^AS_’ for M4sh macros. It also reserves the shell
and environment variable namespace ‘^as_’, and the here-document
delimiter namespace ‘^_AS[A-Z]’ in the output file. You should not
define your own macros or output shell code that conflicts with these
namespaces.

9.1 Common Shell Constructs

M4sh provides portable alternatives for some common shell constructs
that unfortunately are not portable in practice.

— Macro: AS_BOX (text, [char = ‘-’])

Expand into shell code that will output text surrounded by a box
with char in the top and bottom border. text should not
contain a newline, but may contain shell expansions valid for unquoted
here-documents. char defaults to ‘-’, but can be any
character except ‘/’, ‘'’, ‘"’, ‘\’,
‘&’, or ‘`’. This is useful for outputting a comment box into
log files to separate distinct phases of script operation.

— Macro: AS_CASE (word, [pattern1], [if-matched1], ..., [default])

Expand into a shell ‘case’ statement, where word is matched
against one or more patterns. if-matched is run if the
corresponding pattern matched word, else default is run.
Avoids several portability issues (see Limitations of Shell Builtins).

— Macro: AS_DIRNAME (file-name)

Output the directory portion of file-name. For example,
if $file is ‘/one/two/three’, the command
dir=`AS_DIRNAME(["$file"])` sets dir to ‘/one/two’.

This interface may be improved in the future to avoid forks and losing
trailing newlines.

— Macro: AS_ECHO (word)

Emits word to the standard output, followed by a newline. word
must be a single shell word (typically a quoted string). The bytes of
word are output as-is, even if it starts with "-" or contains "\".
Redirections can be placed outside the macro invocation. This is much
more portable than using echo (see Limitations of Shell Builtins).

— Macro: AS_ECHO_N (word)

Emits word to the standard output, without a following newline.
word must be a single shell word (typically a quoted string) and,
for portability, should not include more than one newline. The bytes of
word are output as-is, even if it starts with "-" or contains "\".
Redirections can be placed outside the macro invocation.

— Macro: AS_ESCAPE (string, [chars = ‘`\"$’])

Expands to string, with any characters in chars escaped with
a backslash (‘\’). chars should be at most four bytes long,
and only contain characters from the set ‘`\"$’; however,
characters may be safely listed more than once in chars for the
sake of syntax highlighting editors. The current implementation expands
string after adding escapes; if string contains macro calls
that in turn expand to text needing shell quoting, you can use
AS_ESCAPE(m4_dquote(m4_expand([string]))).

The default for chars (‘\"$`’) is the set of characters
needing escapes when string will be used literally within double
quotes. One common variant is the set of characters to protect when
string will be used literally within back-ticks or an unquoted
here-document (‘\$`’). Another common variant is ‘""’, which can
be used to form a double-quoted string containing the same expansions
that would have occurred if string were expanded in an unquoted
here-document; however, when using this variant, care must be taken that
string does not use double quotes within complex variable
expansions (such as ‘${foo-`echo "hi"`}’) that would be broken
with improper escapes.

This macro is often used with AS_ECHO. For an example, observe
the output generated by the shell code generated from this snippet:

Emit code to probe whether file is a regular file with executable
permissions (and not a directory with search permissions). The caller
is responsible for quoting file.

— Macro: AS_EXIT ([status = ‘$?’])

Emit code to exit the shell with status, defaulting to ‘$?’.
This macro
works around shells that see the exit status of the command prior to
exit inside a ‘trap 0’ handler (see Limitations of Shell Builtins).

— Macro: AS_IF (test1, [run-if-true1], ..., [run-if-false])

Run shell code test1. If test1 exits with a zero status then
run shell code run-if-true1, else examine further tests. If no test
exits with a zero status, run shell code run-if-false, with
simplifications if either run-if-true1 or run-if-false
is empty. For example,

ensures any required macros of HANDLE_FOO
are expanded before the first test.

— Macro: AS_MKDIR_P (file-name)

Make the directory file-name, including intervening directories
as necessary. This is equivalent to ‘mkdir -p -- file-name’,
except that it is portable to older versions of mkdir that
lack support for the -p option or for the --
delimiter (see Limitations of Usual Tools). Also,
AS_MKDIR_P
succeeds if file-name is a symbolic link to an existing directory,
even though Posix is unclear whether ‘mkdir -p’ should
succeed in that case. If creation of file-name fails, exit the
script.

Emit shell code to set the value of ‘$?’ to status, as
efficiently as possible. However, this is not guaranteed to abort a
shell running with set -e (see Limitations of Shell Builtins). This should also be used at the end of a complex shell
function instead of ‘return’ (see Shell Functions) to avoid
a DJGPP shell bug.

— Macro: AS_TR_CPP (expression)

Transform expression into a valid right-hand side for a C #define.
For example:

Transform expression into shell code that generates a valid shell
variable name. The result is literal when possible at m4 time, but must
be used with eval if expression causes shell indirections.
For example:

Compare two strings version-1 and version-2, possibly
containing shell variables, as version strings, and expand
action-if-less, action-if-equal, or action-if-greater
depending upon the result.
The algorithm to compare is similar to the one used by strverscmp in
glibc (see String/Array Comparison).

9.2 Support for indirect variable names

Often, it is convenient to write a macro that will emit shell code
operating on a shell variable. The simplest case is when the variable
name is known. But a more powerful idiom is writing shell code that can
work through an indirection, where another variable or command
substitution produces the name of the variable to actually manipulate.
M4sh supports the notion of polymorphic shell variables, making it easy
to write a macro that can deal with either literal or indirect variable
names and output shell code appropriate for both use cases. Behavior is
undefined if expansion of an indirect variable does not result in a
literal variable name.

If the expansion of expression is definitely a shell literal,
expand if-literal. If the expansion of expression looks
like it might contain shell indirections (such as $var or
`expr`), then if-not is expanded. Sometimes, it is
possible to output optimized code if expression consists only of
shell variable expansions (such as ${var}), in which case
if-simple-ref can be provided; but defaulting to if-not
should always be safe. AS_LITERAL_WORD_IF only expands
if-literal if expression looks like a single shell word,
containing no whitespace; while AS_LITERAL_IF allows whitespace
in expression.

In order to reduce the time spent recognizing whether an
expression qualifies as a literal or a simple indirection, the
implementation is somewhat conservative: expression must be a
single shell word (possibly after stripping whitespace), consisting only
of bytes that would have the same meaning whether unquoted or enclosed
in double quotes (for example, ‘a.b’ results in if-literal,
even though it is not a valid shell variable name; while both ‘'a'’
and ‘[$]’ result in if-not, because they behave differently
than ‘"'a'"’ and ‘"[$]"’). This macro can be used in contexts
for recognizing portable file names (such as in the implementation of
AC_LIBSOURCE), or coupled with some transliterations for forming
valid variable names (such as in the implementation of AS_TR_SH,
which uses an additional m4_translit to convert ‘.’ to
‘_’).

This example shows how to read the contents of the shell variable
bar, exercising all three arguments to AS_LITERAL_IF. It
results in a script that will output the line ‘hello’ three times.

Emit shell code to append the shell expansion of text to the end
of the current contents of the polymorphic shell variable var,
taking advantage of shells that provide the ‘+=’ extension for more
efficient scaling.

For situations where the final contents of var are relatively
short (less than 256 bytes), it is more efficient to use the simpler
code sequence of var=${var}text (or its
polymorphic equivalent of AS_VAR_COPY([t], [var]) and
AS_VAR_SET([var], ["$t"text])). But in the case
when the script will be repeatedly appending text into var,
issues of scaling start to become apparent. A naive implementation
requires execution time linear to the length of the current contents of
var as well as the length of text for a single append, for
an overall quadratic scaling with multiple appends. This macro takes
advantage of shells which provide the extension
var+=text, which can provide amortized constant time
for a single append, for an overall linear scaling with multiple
appends. Note that unlike AS_VAR_SET, this macro requires that
text be quoted properly to avoid field splitting and file name
expansion.

— Macro: AS_VAR_ARITH (var, expression)

Emit shell code to compute the arithmetic expansion of expression,
assigning the result as the contents of the polymorphic shell variable
var. The code takes advantage of shells that provide ‘$(())’
for fewer forks, but uses expr as a fallback. Therefore, the
syntax for a valid expression is rather limited: all operators
must occur as separate shell arguments and with proper quoting, there is
no portable equality operator, all variables containing numeric values
must be expanded prior to the computation, all numeric values must be
provided in decimal without leading zeroes, and the first shell argument
should not be a negative number. In the following example, this snippet
will print ‘(2+3)*4 == 20’.

Output a shell conditional statement. If the contents of the
polymorphic shell variable var match the string word,
execute if-equal; otherwise execute if-not-equal. word
must be a single shell word (typically a quoted string). Avoids
shell bugs if an interrupt signal arrives while a command substitution
in var is being expanded.

A common M4sh idiom involves composing shell variable names from an m4
argument (for example, writing a macro that uses a cache variable).
value can be an arbitrary string, which will be transliterated
into a valid shell name by AS_TR_SH. In order to access the
composed variable name based on value, it is easier to declare a
temporary m4 macro m4-name with AS_VAR_PUSHDEF, then use
that macro as the argument to subsequent AS_VAR macros as a
polymorphic variable name, and finally free the temporary macro with
AS_VAR_POPDEF. These macros are often followed with dnl,
to avoid excess newlines in the output.

Here is an involved example, that shows the power of writing macros that
can handle composed shell variable names:

In the above example, MY_CHECK_HEADER can operate on polymorphic
variable names. In the first invocation, the m4 argument is
stdint.h, which transliterates into a literal stdint_h.
As a result, the temporary macro my_Header expands to the literal
shell name ‘ac_cv_header_stdint_h’. In the second invocation, the
m4 argument to MY_CHECK_HEADER is $header, and the
temporary macro my_Header expands to the indirect shell name
‘$as_my_Header’. During the shell execution of the for loop, when
‘$header’ contains ‘inttypes.h’, then ‘$as_my_Header’
contains ‘ac_cv_header_inttypes_h’. If this script is then run on a
platform where all three headers have been previously detected, the
output of the script will include:

Emit shell code to assign the contents of the polymorphic shell variable
var to the shell expansion of value. value is not
subject to field splitting or file name expansion, so if command
substitution is used, it may be done with ‘`""`’ rather than using
an intermediate variable (see Shell Substitutions). However,
value does undergo rescanning for additional macro names; behavior
is unspecified if late expansion results in any shell meta-characters.

— Macro: AS_VAR_SET_IF (var, [if-set], [if-undef])

Emit a shell conditional statement, which executes if-set if the
polymorphic shell variable var is set to any value, and
if-undef otherwise.

— Macro: AS_VAR_TEST_SET (var)

Emit a shell statement that results in a successful exit status only if
the polymorphic shell variable var is set.

9.3 Initialization Macros

— Macro: AS_BOURNE_COMPATIBLE

Set up the shell to be more compatible with the Bourne shell as
standardized by Posix, if possible. This may involve setting
environment variables, or setting options, or similar
implementation-specific actions. This macro is deprecated, since
AS_INIT already invokes it.

— Macro: AS_INIT

Initialize the M4sh environment. This macro calls m4_init, then
outputs the #! /bin/sh line, a notice about where the output was
generated from, and code to sanitize the environment for the rest of the
script. Among other initializations, this sets SHELL to the shell
chosen to run the script (see CONFIG_SHELL), and LC_ALL to
ensure the C locale. Finally, it changes the current diversion to
BODY. AS_INIT is called automatically by AC_INIT
and AT_INIT, so shell code in configure,
config.status, and testsuite all benefit from a sanitized
shell environment.

— Macro: AS_INIT_GENERATED (file, [comment])

Emit shell code to start the creation of a subsidiary shell script in
file, including changing file to be executable. This macro
populates the child script with information learned from the parent
(thus, the emitted code is equivalent in effect, but more efficient,
than the code output by AS_INIT, AS_BOURNE_COMPATIBLE, and
AS_SHELL_SANITIZE). If present, comment is output near the
beginning of the child, prior to the shell initialization code, and is
subject to parameter expansion, command substitution, and backslash
quote removal. The
parent script should check the exit status after this macro, in case
file could not be properly created (for example, if the disk was
full). If successfully created, the parent script can then proceed to
append additional M4sh constructs into the child script.

Note that the child script starts life without a log file open, so if
the parent script uses logging (see AS_MESSAGE_LOG_FD), you
must temporarily disable any attempts to use the log file until after
emitting code to open a log within the child. On the other hand, if the
parent script has AS_MESSAGE_FD redirected somewhere besides
‘1’, then the child script already has code that copies stdout to
that descriptor. Currently, the suggested
idiom for writing a M4sh shell script from within another script is:

This, however, may change in the future as the M4sh interface is
stabilized further.

Also, be aware that use of LINENO within the child script may
report line numbers relative to their location in the parent script,
even when using AS_LINENO_PREPARE, if the parent script was
unable to locate a shell with working LINENO support.

— Macro: AS_LINENO_PREPARE

Find a shell that supports the special variable LINENO, which
contains the number of the currently executing line. This macro is
automatically invoked by AC_INIT in configure scripts.

— Macro: AS_ME_PREPARE

Set up variable as_me to be the basename of the currently executing
script. This macro is automatically invoked by AC_INIT in
configure scripts.

— Macro: AS_TMPDIR (prefix, [dir = ‘${TMPDIR:=/tmp}’])

Create, as safely as possible, a temporary sub-directory within
dir with a name starting with prefix. prefix should
be 2-4 characters, to make it slightly easier to identify the owner of
the directory. If dir is omitted, then the value of TMPDIR
will be used (defaulting to ‘/tmp’). On success, the name of the
newly created directory is stored in the shell variable tmp. On
error, the script is aborted.

Typically, this macro is coupled with some exit traps to delete the created
directory and its contents on exit or interrupt. However, there is a
slight window between when the directory is created and when the name is
actually known to the shell, so an interrupt at the right moment might
leave the temporary directory behind. Hence it is important to use a
prefix that makes it easier to determine if a leftover temporary
directory from an interrupted script is safe to delete.

The use of the output variable ‘$tmp’ rather than something in the
‘as_’ namespace is historical; it has the unfortunate consequence
that reusing this otherwise common name for any other purpose inside
your script has the potential to break any cleanup traps designed to
remove the temporary directory.

— Macro: AS_SHELL_SANITIZE

Initialize the shell suitably for configure scripts. This has
the effect of AS_BOURNE_COMPATIBLE, and sets some other
environment variables for predictable results from configuration tests.
For example, it sets LC_ALL to change to the default C locale.
See Special Shell Variables. This macro is deprecated, since
AS_INIT already invokes it.

However doing so is seldom needed, because Autoconf provides higher
level macros as described below.

— Macro: AS_MESSAGE_FD

The file descriptor for ‘checking for...’ messages and results.
By default, AS_INIT sets this to ‘1’ for standalone M4sh
clients. However, AC_INIT shuffles things around to another file
descriptor, in order to allow the -q option of
configure to choose whether messages should go to the script's
standard output or be discarded.

If you want to display some messages, consider using one of the printing
macros (see Printing Messages) instead. Copies of messages output
via these macros are also recorded in config.log.

— Macro: AS_MESSAGE_LOG_FD

This must either be empty, or expand to a file descriptor for log
messages. By default, AS_INIT sets this macro to the empty
string for standalone M4sh clients, thus disabling logging. However,
AC_INIT shuffles things around so that both configure
and config.status use config.log for log messages.
Macros that run tools, like AC_COMPILE_IFELSE (see Running the Compiler), redirect all output to this descriptor. You may want to do
so if you develop such a low-level macro.

— Macro: AS_ORIGINAL_STDIN_FD

This must expand to a file descriptor for the original standard input.
By default, AS_INIT sets this macro to ‘0’ for standalone
M4sh clients. However, AC_INIT shuffles things around for
safety.

When configure runs, it may accidentally execute an
interactive command that has the same name as the non-interactive meant
to be used or checked. If the standard input was the terminal, such
interactive programs would cause configure to stop, pending
some user input. Therefore configure redirects its standard
input from /dev/null during its initialization. This is not
normally a problem, since configure normally does not need
user input.

In the extreme case where your configure script really needs to
obtain some values from the original standard input, you can read them
explicitly from AS_ORIGINAL_STDIN_FD.

10 Writing Autoconf Macros

When you write a feature test that could be applicable to more than one
software package, the best thing to do is encapsulate it in a new macro.
Here are some instructions and guidelines for writing Autoconf macros.

10.1 Macro Definitions

— Macro: AC_DEFUN (name, [body])

Autoconf macros are defined using the AC_DEFUN macro, which is
similar to the M4 builtin m4_define macro; this creates a macro
named name and with body as its expansion. In addition to
defining a macro, AC_DEFUN adds to it some code that is used to
constrain the order in which macros are called, while avoiding redundant
output (see Prerequisite Macros).

An Autoconf macro definition looks like this:

AC_DEFUN(macro-name, macro-body)

You can refer to any arguments passed to the macro as ‘$1’,
‘$2’, etc. See How to define new macros, for more complete information on writing M4 macros.

Most macros fall in one of two general categories. The first category
includes macros which take arguments, in order to generate output
parameterized by those arguments. Macros in this category are designed
to be directly expanded, often multiple times, and should not be used as
the argument to AC_REQUIRE. The other category includes macros
which are shorthand for a fixed block of text, and therefore do not take
arguments. For this category of macros, directly expanding the macro
multiple times results in redundant output, so it is more common to use
the macro as the argument to AC_REQUIRE, or to declare the macro
with AC_DEFUN_ONCE (see One-Shot Macros).

Be sure to properly quote both the macro-bodyand the
macro-name to avoid any problems if the macro happens to have
been previously defined.

Each macro should have a header comment that gives its prototype, and a
brief description. When arguments have default values, display them in
the prototype. For example:

Comments about the macro should be left in the header comment. Most
other comments make their way into configure, so just keep
using ‘#’ to introduce comments.

If you have some special comments about pure M4 code, comments
that make no sense in configure and in the header comment, then
use the builtin dnl: it causes M4 to discard the text
through the next newline.

Keep in mind that dnl is rarely needed to introduce comments;
dnl is more useful to get rid of the newlines following macros
that produce no output, such as AC_REQUIRE.

Public third-party macros need to use AC_DEFUN, and not
m4_define, in order to be found by aclocal
(see Extending aclocal).
Additionally, if it is ever determined that a macro should be made
obsolete, it is easy to convert from AC_DEFUN to AU_DEFUN
in order to have autoupdate assist the user in choosing a
better alternative, but there is no corresponding way to make
m4_define issue an upgrade notice (see AU_DEFUN).

There is another subtle, but important, difference between using
m4_define and AC_DEFUN: only the former is unaffected by
AC_REQUIRE. When writing a file, it is always safe to replace a
block of text with a m4_define macro that will expand to the same
text. But replacing a block of text with an AC_DEFUN macro with
the same content does not necessarily give the same results, because it
changes the location where any embedded but unsatisfied
AC_REQUIRE invocations within the block will be expanded. For an
example of this, see Expanded Before Required.

10.2 Macro Names

All of the public Autoconf macros have all-uppercase names in the
namespace ‘^AC_’ to prevent them from accidentally conflicting with
other text; Autoconf also reserves the namespace ‘^_AC_’ for
internal macros. All shell variables that they use for internal
purposes have mostly-lowercase names starting with ‘ac_’. Autoconf
also uses here-document delimiters in the namespace ‘^_AC[A-Z]’. During
configure, files produced by Autoconf make heavy use of the
file system namespace ‘^conf’.

Since Autoconf is built on top of M4sugar (see Programming in M4sugar) and M4sh (see Programming in M4sh), you must also be aware
of those namespaces (‘^_?\(m4\|AS\)_’). And since
configure.ac is also designed to be scanned by Autoheader,
Autoscan, Autoupdate, and Automake, you should be aware of the
‘^_?A[HNUM]_’ namespaces. In general, you should not use
the namespace of a package that does not own the macro or shell code you
are writing.

To ensure that your macros don't conflict with present or future
Autoconf macros, you should prefix your own macro names and any shell
variables they use with some other sequence. Possibilities include your
initials, or an abbreviation for the name of your organization or
software package. Historically, people have not always followed the
rule of using a namespace appropriate for their package, and this has
made it difficult for determining the origin of a macro (and where to
report bugs about that macro), as well as difficult for the true
namespace owner to add new macros without interference from pre-existing
uses of third-party macros. Perhaps the best example of this confusion
is the AM_GNU_GETTEXT macro, which belongs, not to Automake, but
to Gettext.

Most of the Autoconf macros' names follow a structured naming convention
that indicates the kind of feature check by the name. The macro names
consist of several words, separated by underscores, going from most
general to most specific. The names of their cache variables use the
same convention (see Cache Variable Names, for more information on
them).

The first word of the name after the namespace initials (such as
‘AC_’) usually tells the category
of the feature being tested. Here are the categories used in Autoconf for
specific test macros, the kind of macro that you are more likely to
write. They are also used for cache variables, in all-lowercase. Use
them where applicable; where they're not, invent your own categories.

C

C language builtin features.

DECL

Declarations of C variables in header files.

FUNC

Functions in libraries.

GROUP

Posix group owners of files.

HEADER

Header files.

LIB

C libraries.

PROG

The base names of programs.

MEMBER

Members of aggregates.

SYS

Operating system features.

TYPE

C builtin or declared types.

VAR

C variables in libraries.

After the category comes the name of the particular feature being
tested. Any further words in the macro name indicate particular aspects
of the feature. For example, AC_PROG_CC_STDC checks whether the
C compiler supports ISO Standard C.

An internal macro should have a name that starts with an underscore;
Autoconf internals should therefore start with ‘_AC_’.
Additionally, a macro that is an internal subroutine of another macro
should have a name that starts with an underscore and the name of that
other macro, followed by one or more words saying what the internal
macro does. For example, AC_PATH_X has internal macros
_AC_PATH_X_XMKMF and _AC_PATH_X_DIRECT.

10.3 Reporting Messages

When macros statically diagnose abnormal situations, benign or fatal, it
is possible to make autoconf detect the problem, and refuse to
create configure in the case of an error. The macros in this
section are considered obsolescent, and new code should use M4sugar
macros for this purpose, see Diagnostic Macros.

On the other hand, it is possible to want to detect errors when
configure is run, which are dependent on the environment of
the user rather than the maintainer. For dynamic diagnostics, see
Printing Messages.

— Macro: AC_DIAGNOSE (category, message)

Report message as a warning (or as an error if requested by the
user) if warnings of the category are turned on. This macro is
obsolescent; you are encouraged to use:

m4_warn([category], [message])

instead. See m4_warn, for more details, including valid
category names.

— Macro: AC_WARNING (message)

Report message as a syntax warning. This macro is obsolescent;
you are encouraged to use:

m4_warn([syntax], [message])

instead. See m4_warn, for more details, as well as better
finer-grained categories of warnings (not all problems have to do with
syntax).

— Macro: AC_FATAL (message)

Report a severe error message, and have autoconf die.
This macro is obsolescent; you are encouraged to use:

10.4 Dependencies Between Macros

Some Autoconf macros depend on other macros having been called first in
order to work correctly. Autoconf provides a way to ensure that certain
macros are called if needed and a way to warn the user if macros are
called in an order that might cause incorrect operation.

10.4.1 Prerequisite Macros

A macro that you write might need to use values that have previously
been computed by other macros. For example, AC_DECL_YYTEXT
examines the output of flex or lex, so it depends on
AC_PROG_LEX having been called first to set the shell variable
LEX.

Rather than forcing the user of the macros to keep track of the
dependencies between them, you can use the AC_REQUIRE macro to do
it automatically. AC_REQUIRE can ensure that a macro is only
called if it is needed, and only called once.

— Macro: AC_REQUIRE (macro-name)

If the M4 macro macro-name has not already been called, call it
(without any arguments). Make sure to quote macro-name with
square brackets. macro-name must have been defined using
AC_DEFUN or else contain a call to AC_PROVIDE to indicate
that it has been called.

AC_REQUIRE must be used inside a macro defined by AC_DEFUN; it
must not be called from the top level. Also, it does not make sense to
require a macro that takes parameters.

AC_REQUIRE is often misunderstood. It really implements
dependencies between macros in the sense that if one macro depends upon
another, the latter is expanded before the body of the
former. To be more precise, the required macro is expanded before
the outermost defined macro in the current expansion stack.
In particular, ‘AC_REQUIRE([FOO])’ is not replaced with the body of
FOO. For instance, this definition of macros:

This behavior was chosen on purpose: (i) it prevents messages in
required macros from interrupting the messages in the requiring macros;
(ii) it avoids bad surprises when shell conditionals are used, as in:

if ...; then
AC_REQUIRE([SOME_CHECK])
fi
...
SOME_CHECK

However, this implementation can lead to another class of problems.
Consider the case where an outer macro first expands, then indirectly
requires, an inner macro:

Prior to Autoconf 2.64, the implementation of AC_REQUIRE
recognized that TESTB needed to be hoisted prior to the expansion
of OUTER, but because TESTA had already been directly
expanded, it failed to hoist TESTA. Therefore, the expansion of
TESTB occurs prior to its prerequisites, leading to the following
output:

in B
bug
in OUTER
in A
in C

Newer Autoconf is smart enough to recognize this situation, and hoists
TESTA even though it has already been expanded, but issues a
syntax warning in the process. This is because the hoisted expansion of
TESTA defeats the purpose of using AC_REQUIRE to avoid
redundant code, and causes its own set of problems if the hoisted macro
is not idempotent:

in A
in B
in OUTER
in A
duplicate
in C

The bug is not in Autoconf, but in the macro definitions. If you ever
pass a particular macro name to AC_REQUIRE, then you are implying
that the macro only needs to be expanded once. But to enforce this,
either the macro must be declared with AC_DEFUN_ONCE (although
this only helps in Autoconf 2.64 or newer), or all
uses of that macro should be through AC_REQUIRE; directly
expanding the macro defeats the point of using AC_REQUIRE to
eliminate redundant expansion. In the example, this rule of thumb was
violated because TESTB requires TESTA while OUTER
directly expands it. One way of fixing the bug is to factor
TESTA into two macros, the portion designed for direct and
repeated use (here, named TESTA), and the portion designed for
one-shot output and used only inside AC_REQUIRE (here, named
TESTA_PREREQ). Then, by fixing all clients to use the correct
calling convention according to their needs:

the resulting output will then obey all dependency rules and avoid any
syntax warnings, whether the script is built with old or new Autoconf
versions:

in A_PREREQ
in B
in OUTER
in A
in C

The helper macros AS_IF and AS_CASE may be used to
enforce expansion of required macros outside of shell conditional
constructs. You are furthermore encouraged, although not required, to
put all AC_REQUIRE calls
at the beginning of a macro. You can use dnl to avoid the empty
lines they leave.

10.4.2 Suggested Ordering

Some macros should be run before another macro if both are called, but
neither requires that the other be called. For example, a macro
that changes the behavior of the C compiler should be called before any
macros that run the C compiler. Many of these dependencies are noted in
the documentation.

Autoconf provides the AC_BEFORE macro to warn users when macros
with this kind of dependency appear out of order in a
configure.ac file. The warning occurs when creating
configure from configure.ac, not when running
configure.

For example, AC_PROG_CPP checks whether the C compiler
can run the C preprocessor when given the -E option. It should
therefore be called after any macros that change which C compiler is
being used, such as AC_PROG_CC. So AC_PROG_CC contains:

AC_BEFORE([$0], [AC_PROG_CPP])dnl

This warns the user if a call to AC_PROG_CPP has already occurred
when AC_PROG_CC is called.

— Macro: AC_BEFORE (this-macro-name, called-macro-name)

Make M4 print a warning message to the standard error output if
called-macro-name has already been called. this-macro-name
should be the name of the macro that is calling AC_BEFORE. The
macro called-macro-name must have been defined using
AC_DEFUN or else contain a call to AC_PROVIDE to indicate
that it has been called.

10.4.3 One-Shot Macros

Some macros should be called only once, either because calling them
multiple time is unsafe, or because it is bad style. For instance
Autoconf ensures that AC_CANONICAL_BUILD and cousins
(see Canonicalizing) are evaluated only once, because it makes no
sense to run these expensive checks more than once. Such one-shot
macros can be defined using AC_DEFUN_ONCE.

— Macro: AC_DEFUN_ONCE (macro-name, macro-body)

Declare macro macro-name like AC_DEFUN would (see Macro Definitions), but add additional logic that guarantees that only the
first use of the macro (whether by direct expansion or
AC_REQUIRE) causes an expansion of macro-body; the
expansion will occur before the start of any enclosing macro defined by
AC_DEFUN. Subsequent expansions are silently ignored.
Generally, it does not make sense for macro-body to use parameters
such as $1.

Prior to Autoconf 2.64, a macro defined by AC_DEFUN_ONCE would
emit a warning if it was directly expanded a second time, so for
portability, it is better to use AC_REQUIRE than direct
invocation of macro-name inside a macro defined by AC_DEFUN
(see Prerequisite Macros).

10.5 Obsoleting Macros

Configuration and portability technology has evolved over the years.
Often better ways of solving a particular problem are developed, or
ad-hoc approaches are systematized. This process has occurred in many
parts of Autoconf. One result is that some of the macros are now
considered obsolete; they still work, but are no longer considered
the best thing to do, hence they should be replaced with more modern
macros. Ideally, autoupdate should replace the old macro calls
with their modern implementation.

Autoconf provides a simple means to obsolete a macro.

— Macro: AU_DEFUN (old-macro, implementation, [message])

Define old-macro as implementation. The only difference
with AC_DEFUN is that the user is warned that
old-macro is now obsolete.

If she then uses autoupdate, the call to old-macro is
replaced by the modern implementation. message should
include information on what to do after running autoupdate;
autoupdate prints it as a warning, and includes it
in the updated configure.ac file.

The details of this macro are hairy: if autoconf encounters an
AU_DEFUNed macro, all macros inside its second argument are expanded
as usual. However, when autoupdate is run, only M4 and M4sugar
macros are expanded here, while all other macros are disabled and
appear literally in the updated configure.ac.

— Macro: AU_ALIAS (old-name, new-name)

Used if the old-name is to be replaced by a call to new-macro
with the same parameters. This happens for example if the macro was renamed.

10.6 Coding Style

The Autoconf macros follow a strict coding style. You are encouraged to
follow this style, especially if you intend to distribute your macro,
either by contributing it to Autoconf itself or the
Autoconf Macro Archive, or by other means.

Do not try to invent new interfaces. It is likely that there is a macro
in Autoconf that resembles the macro you are defining: try to stick to
this existing interface (order of arguments, default values, etc.). We
are conscious that some of these interfaces are not perfect;
nevertheless, when harmless, homogeneity should be preferred over
creativity.

Be careful about clashes both between M4 symbols and between shell
variables.

If you stick to the suggested M4 naming scheme (see Macro Names),
you are unlikely to generate conflicts. Nevertheless, when you need to
set a special value, avoid using a regular macro name; rather,
use an “impossible” name. For instance, up to version 2.13, the macro
AC_SUBST used to remember what symbol macros were already defined
by setting AC_SUBST_symbol, which is a regular macro name.
But since there is a macro named AC_SUBST_FILE, it was just
impossible to ‘AC_SUBST(FILE)’! In this case,
AC_SUBST(symbol) or _AC_SUBST(symbol) should
have been used (yes, with the parentheses).

No Autoconf macro should ever enter the user-variable name space; i.e.,
except for the variables that are the actual result of running the
macro, all shell variables should start with ac_. In
addition, small macros or any macro that is likely to be embedded in
other macros should be careful not to use obvious names.

Do not use dnl to introduce comments: most of the comments you
are likely to write are either header comments which are not output
anyway, or comments that should make their way into configure.
There are exceptional cases where you do want to comment special M4
constructs, in which case dnl is right, but keep in mind that it
is unlikely.

M4 ignores the leading blanks and newlines before each argument.
Use this feature to
indent in such a way that arguments are (more or less) aligned with the
opening parenthesis of the macro being called. For instance, instead of

When using AC_RUN_IFELSE or any macro that cannot work when
cross-compiling, provide a pessimistic value (typically ‘no’).

Feel free to use various tricks to prevent auxiliary tools, such as
syntax-highlighting editors, from behaving improperly. For instance,
instead of:

m4_bpatsubst([$1], [$"])

use

m4_bpatsubst([$1], [$""])

so that Emacsen do not open an endless “string” at the first quote.
For the same reasons, avoid:

test $[#] != 0

and use:

test $[@%:@] != 0

Otherwise, the closing bracket would be hidden inside a ‘#’-comment,
breaking the bracket-matching highlighting from Emacsen. Note the
preferred style to escape from M4: ‘$[1]’, ‘$[@]’, etc. Do
not escape when it is unnecessary. Common examples of useless quotation
are ‘[$]$1’ (write ‘$$1’), ‘[$]var’ (use ‘$var’),
etc. If you add portability issues to the picture, you'll prefer
‘${1+"$[@]"}’ to ‘"[$]@"’, and you'll prefer do something
better than hacking Autoconf :-).

When using sed, don't use -e except for indenting
purposes. With the s and y commands, the preferred
separator is ‘/’ unless ‘/’ itself might appear in the pattern
or replacement, in which case you should use ‘|’, or optionally
‘,’ if you know the pattern and replacement cannot contain a file
name. If none of these characters will do, choose a printable character
that cannot appear in the pattern or replacement. Characters from the
set ‘"#$&'()*;<=>?`|~’ are good choices if the pattern or
replacement might contain a file name, since they have special meaning
to the shell and are less likely to occur in file names.

See Macro Definitions, for details on how to define a macro. If a
macro doesn't use AC_REQUIRE, is expected to never be the object
of an AC_REQUIRE directive, and macros required by other macros
inside arguments do not need to be expanded before this macro, then
use m4_define. In case of doubt, use AC_DEFUN.
Also take into account that public third-party macros need to use
AC_DEFUN in order to be found by aclocal
(see Extending aclocal).
All the AC_REQUIRE statements should be at the beginning of the
macro, and each statement should be followed by dnl.

You should not rely on the number of arguments: instead of checking
whether an argument is missing, test that it is not empty. It provides
both a simpler and a more predictable interface to the user, and saves
room for further arguments.

Unless the macro is short, try to leave the closing ‘])’ at the
beginning of a line, followed by a comment that repeats the name of the
macro being defined. This introduces an additional newline in
configure; normally, that is not a problem, but if you want to
remove it you can use ‘[]dnl’ on the last line. You can similarly
use ‘[]dnl’ after a macro call to remove its newline. ‘[]dnl’
is recommended instead of ‘dnl’ to ensure that M4 does not
interpret the ‘dnl’ as being attached to the preceding text or
macro output. For example, instead of:

If the macro is long, try to split it into logical chunks. Typically,
macros that check for a bug in a function and prepare its
AC_LIBOBJ replacement should have an auxiliary macro to perform
this setup. Do not hesitate to introduce auxiliary macros to factor
your code.

In order to highlight the recommended coding style, here is a macro
written the old way:

11 Portable Shell Programming

When writing your own checks, there are some shell-script programming
techniques you should avoid in order to make your code portable. The
Bourne shell and upward-compatible shells like the Korn shell and Bash
have evolved over the years, and many features added to the original
System7 shell are now supported on all interesting porting targets.
However, the following discussion between Russ Allbery and Robert Lipe
is worth reading:

Russ Allbery:

The GNU assumption that /bin/sh is the one and only shell
leads to a permanent deadlock. Vendors don't want to break users'
existing shell scripts, and there are some corner cases in the Bourne
shell that are not completely compatible with a Posix shell. Thus,
vendors who have taken this route will never (OK...“never say
never”) replace the Bourne shell (as /bin/sh) with a
Posix shell.

Robert Lipe:

This is exactly the problem. While most (at least most System V's) do
have a Bourne shell that accepts shell functions most vendor
/bin/sh programs are not the Posix shell.

So while most modern systems do have a shell somewhere that meets the
Posix standard, the challenge is to find it.

For this reason, part of the job of M4sh (see Programming in M4sh)
is to find such a shell. But to prevent trouble, if you're not using
M4sh you should not take advantage of features that were added after Unix
version 7, circa 1977 (see Systemology); you should not use aliases,
negated character classes, or even unset. # comments,
while not in Unix version 7, were retrofitted in the original Bourne
shell and can be assumed to be part of the least common denominator.

On the other hand, if you're using M4sh you can assume that the shell
has the features that were added in SVR2 (circa 1984), including shell
functions,
return, unset, and I/O redirection for builtins. For
more information, refer to http://www.in-ulm.de/~mascheck/bourne/.
However, some pitfalls have to be avoided for portable use of these
constructs; these will be documented in the rest of this chapter.
See in particular Shell Functions and Limitations of Shell Builtins.

Some ancient systems have quite
small limits on the length of the ‘#!’ line; for instance, 32
bytes (not including the newline) on SunOS 4.
However, these ancient systems are no longer of practical concern.

The set of external programs you should run in a configure script
is fairly small. See Utilities in Makefiles, for the list. This
restriction allows users to start out with a fairly small set of
programs and build the rest, avoiding too many interdependencies between
packages.

There are other sources of documentation about shells. The
specification for the Posix
Shell Command Language, though more generous than the restrictive shell
subset described above, is fairly portable nowadays. Also please see
the Shell FAQs.

11.1 Shellology

There are several families of shells, most prominently the Bourne family
and the C shell family which are deeply incompatible. If you want to
write portable shell scripts, avoid members of the C shell family. The
the Shell difference FAQ includes a small history of Posix shells, and a
comparison between several of them.

Below we describe some of the members of the Bourne shell family.

Ash

Ash is often used on GNU/Linux and BSD
systems as a light-weight Bourne-compatible shell. Ash 0.2 has some
bugs that are fixed in the 0.3.x series, but portable shell scripts
should work around them, since version 0.2 is still shipped with many
GNU/Linux distributions.

To be compatible with Ash 0.2:

don't use ‘$?’ after expanding empty or unset variables,
or at the start of an eval:

beware that single builtin substitutions are not performed by a
subshell, hence their effect applies to the current shell! See Shell Substitutions, item “Command Substitution”.

Bash

To detect whether you are running Bash, test whether
BASH_VERSION is set. To require
Posix compatibility, run ‘set -o posix’. See Bash Posix Mode, for details.

Bash 2.05 and later

Versions 2.05 and later of Bash use a different format for the
output of the set builtin, designed to make evaluating its
output easier. However, this output is not compatible with earlier
versions of Bash (or with many other shells, probably). So if
you use Bash 2.05 or higher to execute configure,
you'll need to use Bash 2.05 for all other build tasks as well.

Ksh

The Korn shell is compatible with the Bourne family and it mostly
conforms to Posix. It has two major variants commonly
called ‘ksh88’ and ‘ksh93’, named after the years of initial
release. It is usually called ksh, but is called sh
on some hosts if you set your path appropriately.

Solaris systems have three variants:
/usr/bin/ksh is ‘ksh88’; it is
standard on Solaris 2.0 and later.
/usr/xpg4/bin/sh is a Posix-compliant variant of
‘ksh88’; it is standard on Solaris 9 and later.
/usr/dt/bin/dtksh is ‘ksh93’.
Variants that are not standard may be parts of optional
packages. There is no extra charge for these packages, but they are
not part of a minimal OS install and therefore some installations may
not have it.

Starting with Tru64 Version 4.0, the Korn shell /usr/bin/ksh
is also available as /usr/bin/posix/sh. If the environment
variable BIN_SH is set to xpg4, subsidiary invocations of
the standard shell conform to Posix.

Pdksh

A public-domain clone of the Korn shell called pdksh is widely
available: it has most of the ‘ksh88’ features along with a few of
its own. It usually sets KSH_VERSION, except if invoked as
/bin/sh on OpenBSD, and similarly to Bash you can require
Posix compatibility by running ‘set -o posix’. Unfortunately, with
pdksh 5.2.14 (the latest stable version as of January 2007)
Posix mode is buggy and causes pdksh to depart from Posix in
at least one respect, see Shell Substitutions.

Zsh

To detect whether you are running zsh, test whether
ZSH_VERSION is set. By default zsh is not
compatible with the Bourne shell: you must execute ‘emulate sh’,
and for zsh versions before 3.1.6-dev-18 you must also
set NULLCMD to ‘:’. See Compatibility, for details.

The default Mac OS X sh was originally Zsh; it was changed to
Bash in Mac OS X 10.2.

11.2 Invoking the Shell

The Korn shell (up to at least version M-12/28/93d) has a bug when
invoked on a file whose name does not contain a slash. It first
searches for the file's name in PATH, and if found it executes
that rather than the original file. For example, assuming there is a
binary executable /usr/bin/script in your PATH, the last
command in the following example fails because the Korn shell finds
/usr/bin/script and refuses to execute it as a shell script:

Bash 2.03 has a bug when invoked with the -c option: if the
option-argument ends in backslash-newline, Bash incorrectly reports a
syntax error. The problem does not occur if a character follows the
backslash:

11.3 Here-Documents

Don't rely on ‘\’ being preserved just because it has no special
meaning together with the next symbol. In the native sh
on OpenBSD 2.7 ‘\"’ expands to ‘"’ in here-documents with
unquoted delimiter. As a general rule, if ‘\\’ expands to ‘\’
use ‘\\’ to get ‘\’.

With OpenBSD 2.7's sh

$ cat <<EOF
> \" \\
> EOF
" \

and with Bash:

bash-2.04$ cat <<EOF
> \" \\
> EOF
\" \

Using command substitutions in a here-document that is fed to a shell
function is not portable. For example, with Solaris 10 /bin/sh:

Some shells mishandle large here-documents: for example,
Solaris 10 dtksh and the UnixWare 7.1.1 Posix shell, which are
derived from Korn shell version M-12/28/93d, mishandle braced variable
expansion that crosses a 1024- or 4096-byte buffer boundary
within a here-document. Only the part of the variable name after the boundary
is used. For example, ${variable} could be replaced by the expansion
of ${ble}. If the end of the variable name is aligned with the block
boundary, the shell reports an error, as if you used ${}.
Instead of ${variable-default}, the shell may expand
${riable-default}, or even ${fault}. This bug can often
be worked around by omitting the braces: $variable. The bug was
fixed in
‘ksh93g’ (1998-04-30) but as of 2006 many operating systems were
still shipping older versions with the bug.

Empty here-documents are not portable either; with the following code,
zsh up to at least version 4.3.10 creates a file with a single
newline, whereas other shells create an empty file:

cat >file <<EOF
EOF

Many shells (including the Bourne shell) implement here-documents
inefficiently. In particular, some shells can be extremely inefficient when
a single statement contains many here-documents. For instance if your
configure.ac includes something like:

if <cross_compiling>; then
assume this and that
else
check this
check that
check something else
...
on and on forever
...
fi

A shell parses the whole if/fi construct, creating
temporary files for each here-document in it. Some shells create links
for such here-documents on every fork, so that the clean-up code
they had installed correctly removes them. It is creating the links
that can take the shell forever.

Moving the tests out of the if/fi, or creating multiple
if/fi constructs, would improve the performance
significantly. Anyway, this kind of construct is not exactly the
typical use of Autoconf. In fact, it's even not recommended, because M4
macros can't look into shell conditionals, so we may fail to expand a
macro when it was expanded before in a conditional path, and the
condition turned out to be false at runtime, and we end up not
executing the macro at all.

Be careful with the use of ‘<<-’ to unindent here-documents. The
behavior is only portable for stripping leading <TAB>s, and things
can silently break if an overzealous editor converts to using leading
spaces (not all shells are nice enough to warn about unterminated
here-documents).

11.4 File Descriptors

Most shells, if not all (including Bash, Zsh, Ash), output traces on
stderr, even for subshells. This might result in undesirable content
if you meant to capture the standard-error output of the inner command:

One workaround is to grep out uninteresting lines, hoping not to remove
good ones.

If you intend to redirect both standard error and standard output,
redirect standard output first. This works better with HP-UX,
since its shell mishandles tracing if standard error is redirected
first:

$ sh -x -c ': 2>err >out'
+ :
+ 2> err $ cat err
1> out

Don't try to redirect the standard error of a command substitution. It
must be done inside the command substitution. When running
‘: `cd /zorglub` 2>/dev/null’ expect the error message to
escape, while ‘: `cd /zorglub 2>/dev/null`’ works properly.

On the other hand, some shells, such as Solaris or FreeBSD
/bin/sh, warn about missing programs before performing
redirections. Therefore, to silently check whether a program exists, it
is necessary to perform redirections on a subshell or brace group:

FreeBSD 6.2 sh may mix the trace output lines from the statements in a
shell pipeline.

It is worth noting that Zsh (but not Ash nor Bash) makes it possible
in assignments though: ‘foo=`cd /zorglub` 2>/dev/null’.

Some shells, like ash, don't recognize bi-directional
redirection (‘<>’). And even on shells that recognize it, it is
not portable to use on fifos: Posix does not require read-write support
for named pipes, and Cygwin does not support it:

Don't rely on file descriptors 0, 1, and 2 remaining closed in a
subsidiary program. If any of these descriptors is closed, the
operating system may open an unspecified file for the descriptor in the
new process image. Posix 2008 says this may be done only if the
subsidiary program is set-user-ID or set-group-ID, but HP-UX 11.23 does
it even for ordinary programs, and the next version of Posix will allow
HP-UX behavior.

If you want a file descriptor above 2 to be inherited into a child
process, then you must use redirections specific to that command or a
containing subshell or command group, rather than relying on
exec in the shell. In ksh as well as HP-UX
sh, file descriptors above 2 which are opened using
‘exec n>file’ are closed by a subsequent ‘exec’ (such as
that involved in the fork-and-exec which runs a program or script):

Don't rely on duplicating a closed file descriptor to cause an
error. With Solaris /bin/sh, failed duplication is silently
ignored, which can cause unintended leaks to the original file
descriptor. In this example, observe the leak to standard output:

Fortunately, an attempt to close an already closed file descriptor will
portably succeed. Likewise, it is safe to use either style of
‘n<&-’ or ‘n>&-’ for closing a file descriptor,
even if it doesn't match the read/write mode that the file descriptor
was opened with.

DOS variants cannot rename or remove open files, such as in
‘mv foo bar >foo’ or ‘rm foo >foo’, even though this is
perfectly portable among Posix hosts.

A few ancient systems reserved some file descriptors. By convention,
file descriptor 3 was opened to /dev/tty when you logged into
Eighth Edition (1985) through Tenth Edition Unix (1989). File
descriptor 4 had a special use on the Stardent/Kubota Titan (circa
1990), though we don't now remember what it was. Both these systems are
obsolete, so it's now safe to treat file descriptors 3 and 4 like any
other file descriptors.

On the other hand, you can't portably use multi-digit file descriptors.
Solaris ksh doesn't understand any file descriptor larger than
‘9’:

11.5 Signal Handling

Portable handling of signals within the shell is another major source of
headaches. This is worsened by the fact that various different, mutually
incompatible approaches are possible in this area, each with its
distinctive merits and demerits. A detailed description of these possible
approaches, as well as of their pros and cons, can be found in
this article.

Solaris 10 /bin/sh automatically traps most signals by default;
the shell still exits with error upon termination by one of those signals,
but in such a case the exit status might be somewhat unexpected (even if
allowed by POSIX, strictly speaking):

This gets even worse if one is using the POSIX `wait' interface to get
details about the shell process terminations: it will result in the shell
having exited normally, rather than by receiving a signal.

Various shells seem to handle SIGQUIT specially: they ignore it even
if it is not blocked, and even if the shell is not running interactively
(in fact, even if the shell has no attached tty); among these shells
are at least Bash (from version 2 onwards), Zsh 4.3.12, Solaris 10
/bin/ksh and /usr/xpg4/bin/sh, and AT&T ksh93 (2011).
Still, SIGQUIT seems to be trappable quite portably within all
these shells. OTOH, some other shells doesn't special-case the handling
of SIGQUIT; among these shells are at least pdksh 5.2.14,
Solaris 10 and NetBSD 5.1 /bin/sh, and the Almquist Shell 0.5.5.1.

Some shells (especially Korn shells and derivatives) might try to
propagate to themselves a signal that has killed a child process; this is
not a bug, but a conscious design choice (although its overall value might
be debatable). The exact details of how this is attained vary from shell
to shell. For example, upon running perl -e 'kill 2, $$', after
the perl process has been interrupted AT&T ksh93 (2011) will
proceed to send itself a SIGINT, while Solaris 10 /bin/ksh
and /usr/xpg4/bin/sh will proceed to exit with status 130 (i.e.,
128 + 2). In any case, if there is an active trap associated with
SIGINT, those shells will correctly execute it.

Some Korn shells, when a child process die due receiving a signal with
signal number n, can leave in ‘$?’ an exit status of
256+n instead of the more common 128+n. Observe the
difference between AT&T ksh93 (2011) and bash 4.1.5 on
Debian:

This ksh behavior is allowed by POSIX, if implemented with
due care; see this Austin Group discussion for more background. However, if it is not
implemented with proper care, such a behavior might cause problems
in some corner cases. To see why, assume we have a “wrapper” script
like this:

If wrapped_command is interrupted by a SIGHUP (which
has signal number 1), ret will be set to 257. Unless the
exit shell builtin is smart enough to understand that such
a value can only have originated from a signal, and adjust the final
wait status of the shell appropriately, the value 257 will just get
truncated to 1 by the closing exit call, so that a caller of
the script will have no way to determine that termination by a signal
was involved. Observe the different behavior of AT&T ksh93
(2011) and bash 4.1.5 on Debian:

11.6 File System Conventions

Autoconf uses shell-script processing extensively, so the file names
that it processes should not contain characters that are special to the
shell. Special characters include space, tab, newline, NUL, and
the following:

" # $ & ' ( ) * ; < = > ? [ \ ` |

Also, file names should not begin with ‘~’ or ‘-’, and should
contain neither ‘-’ immediately after ‘/’ nor ‘~’
immediately after ‘:’. On Posix-like platforms, directory names
should not contain ‘:’, as this runs afoul of ‘:’ used as the
path separator.

These restrictions apply not only to the files that you distribute, but
also to the absolute file names of your source, build, and destination
directories.

On some Posix-like platforms, ‘!’ and ‘^’ are special too, so
they should be avoided.

Posix lets implementations treat leading // specially, but
requires leading /// and beyond to be equivalent to /.
Most Unix variants treat // like /. However, some treat
// as a “super-root” that can provide access to files that are
not otherwise reachable from /. The super-root tradition began
with Apollo Domain/OS, which died out long ago, but unfortunately Cygwin
has revived it.

While autoconf and friends are usually run on some Posix
variety, they can be used on other systems, most notably DOS
variants. This impacts several assumptions regarding file names.

For example, the following code:

case $foo_dir in
/*) # Absolute
;;
*)
foo_dir=$dots$foo_dir ;;
esac

fails to properly detect absolute file names on those systems, because
they can use a drivespec, and usually use a backslash as directory
separator. If you want to be portable to DOS variants (at the
price of rejecting valid but oddball Posix file names like a:\b),
you can check for absolute file names like this:

Also, because the colon is used as part of a drivespec, these systems don't
use it as path separator. When creating or accessing paths, you can use the
PATH_SEPARATOR output variable instead. configure sets this
to the appropriate value for the build system (‘:’ or ‘;’) when it
starts up.

File names need extra care as well. While DOS variants
that are Posixy enough to run autoconf (such as DJGPP)
are usually able to handle long file names properly, there are still
limitations that can seriously break packages. Several of these issues
can be easily detected by the
doschk
package.

A short overview follows; problems are marked with SFN/LFN to
indicate where they apply: SFN means the issues are only relevant to
plain DOS, not to DOS under Microsoft Windows
variants, while LFN identifies problems that exist even under
Microsoft Windows variants.

No multiple dots (SFN)

DOS cannot handle multiple dots in file names. This is an especially
important thing to remember when building a portable configure script,
as autoconf uses a .in suffix for template files.

DOS cannot handle file names that start with a dot. This is usually
not important for autoconf.

Case insensitivity (LFN)

DOS is case insensitive, so you cannot, for example, have both a
file called ‘INSTALL’ and a directory called ‘install’. This
also affects make; if there's a file called ‘INSTALL’ in
the directory, ‘make install’ does nothing (unless the
‘install’ target is marked as PHONY).

The 8+3 limit (SFN)

Because the DOS file system only stores the first 8 characters of
the file name and the first 3 of the extension, those must be unique.
That means that foobar-part1.c, foobar-part2.c and
foobar-prettybird.c all resolve to the same file name
(FOOBAR-P.C). The same goes for foo.bar and
foo.bartender.

The 8+3 limit is not usually a problem under Microsoft Windows, as it
uses numeric
tails in the short version of file names to make them unique. However, a
registry setting can turn this behavior off. While this makes it
possible to share file trees containing long file names between SFN
and LFN environments, it also means the above problem applies there
as well.

Invalid characters (LFN)

Some characters are invalid in DOS file names, and should therefore
be avoided. In a LFN environment, these are ‘/’, ‘\’,
‘?’, ‘*’, ‘:’, ‘<’, ‘>’, ‘|’ and ‘"’.
In a SFN environment, other characters are also invalid. These
include ‘+’, ‘,’, ‘[’ and ‘]’.

Invalid names (LFN)

Some DOS file names are reserved, and cause problems if you
try to use files with those names. These names include CON,
AUX, COM1, COM2, COM3, COM4,
LPT1, LPT2, LPT3, NUL, and PRN.
File names are case insensitive, so even names like
aux/config.guess are disallowed.

11.7 Shell Pattern Matching

Nowadays portable patterns can use negated character classes like
‘[!-aeiou]’. The older syntax ‘[^-aeiou]’ is supported by
some shells but not others; hence portable scripts should never use
‘^’ as the first character of a bracket pattern.

Outside the C locale, patterns like ‘[a-z]’ are problematic since
they may match characters that are not lower-case letters.

11.8 Shell Substitutions

Contrary to a persistent urban legend, the Bourne shell does not
systematically split variables and back-quoted expressions, in particular
on the right-hand side of assignments and in the argument of case.
For instance, the following code:

and in fact it is even more portable: in the first case of the
first attempt, the computation of top_srcdir is not portable,
since not all shells properly understand "`..."..."...`",
for example Solaris 10 ksh:

Posix does not specify behavior for this sequence. On the other hand,
behavior for "`...\"...\"...`" is specified by Posix,
but in practice, not all shells understand it the same way: pdksh 5.2.14
prints spurious quotes when in Posix mode:

There is just no portable way to use double-quoted strings inside
double-quoted back-quoted expressions (pfew!).

Bash 4.1 has a bug where quoted empty strings adjacent to unquoted
parameter expansions are elided during word splitting. Meanwhile, zsh
does not perform word splitting except when in Bourne compatibility
mode. In the example below, the correct behavior is to have five
arguments to the function, and exactly two spaces on either side of the
middle ‘-’, since word splitting collapses multiple spaces in
‘$f’ but leaves empty arguments intact.

You can work around this by doing manual word splitting, such as using
‘"$str" $list’ rather than ‘"$str"$list’.

There are also portability pitfalls with particular expansions:

$@

One of the most famous shell-portability issues is related to
‘"$@"’. When there are no positional arguments, Posix says
that ‘"$@"’ is supposed to be equivalent to nothing, but the
original Unix version 7 Bourne shell treated it as equivalent to
‘""’ instead, and this behavior survives in later implementations
like Digital Unix 5.0.

The traditional way to work around this portability problem is to use
‘${1+"$@"}’. Unfortunately this method does not work with
Zsh (3.x and 4.x), which is used on Mac OS X. When emulating
the Bourne shell, Zsh performs word splitting on ‘${1+"$@"}’:

zsh $ emulate sh
zsh $ for i in "$@"; do echo $i; done
Hello World
!
zsh $ for i in ${1+"$@"}; do echo $i; done
Hello
World
!

Zsh handles plain ‘"$@"’ properly, but we can't use plain
‘"$@"’ because of the portability problems mentioned above.
One workaround relies on Zsh's “global aliases” to convert
‘${1+"$@"}’ into ‘"$@"’ by itself:

test "${ZSH_VERSION+set}" = set && alias -g '${1+"$@"}'='"$@"'

Zsh only recognizes this alias when a shell word matches it exactly;
‘"foo"${1+"$@"}’ remains subject to word splitting. Since this
case always yields at least one shell word, use plain ‘"$@"’.

A more conservative workaround is to avoid ‘"$@"’ if it is
possible that there may be no positional arguments. For example,
instead of:

cat conftest.c "$@"

you can use this instead:

case $# in
0) cat conftest.c;;
*) cat conftest.c "$@";;
esac

Autoconf macros often use the set command to update
‘$@’, so if you are writing shell code intended for
configure you should not assume that the value of ‘$@’
persists for any length of time.

${10}

The 10th, 11th, ... positional parameters can be accessed only after
a shift. The 7th Edition shell reported an error if given
${10}, and
Solaris 10 /bin/sh still acts that way:

$ set 1 2 3 4 5 6 7 8 9 10
$ echo ${10}
bad substitution

Conversely, not all shells obey the Posix rule that when braces are
omitted, multiple digits beyond a ‘$’ imply the single-digit
positional parameter expansion concatenated with the remaining literal
digits. To work around the issue, you must use braces.

Old BSD shells, including the Ultrix sh, don't accept the
colon for any shell substitution, and complain and die.
Similarly for ${var:=value}, ${var:?value}, etc.
However, all shells that support functions allow the use of colon in
shell substitution, and since m4sh requires functions, you can portably
use null variable substitution patterns in configure scripts.

${var+value}

When using ‘${var-value}’ or
‘${var-value}’ for providing alternate substitutions,
value must either be a single shell word, quoted, or in the
context of an unquoted here-document. Solaris
/bin/sh complains otherwise.

According to Posix, if an expansion occurs inside double quotes, then
the use of unquoted double quotes within value is unspecified, and
any single quotes become literal characters; in that case, escaping must
be done with backslash. Likewise, the use of unquoted here-documents is
a case where double quotes have unspecified results:

Perhaps the easiest way to work around quoting issues in a manner
portable to all shells is to place the results in a temporary variable,
then use ‘$t’ as the value, rather than trying to inline
the expression needing quoting.

When using ‘${var=value}’ to assign a default value
to var, remember that even though the assignment to var does
not undergo file name expansion, the result of the variable expansion
does unless the expansion occurred within double quotes. In particular,
when using : followed by unquoted variable expansion for the
side effect of setting a default value, if the final value of
‘$var’ contains any globbing characters (either from value or
from prior contents), the shell has to spend time performing file name
expansion and field splitting even though those results will not be
used. Therefore, it is a good idea to consider double quotes when performing
default initialization; while remembering how this impacts any quoting
characters appearing in value.

otherwise some shells, such as Solaris /bin/sh or on Digital
Unix V 5.0, die because of a “bad substitution”. Meanwhile, Posix
requires that with ‘=’, quote removal happens prior to the
assignment, and the expansion be the final contents of var without
quoting (and thus subject to field splitting), in contrast to the
behavior with ‘-’ passing the quoting through to the final
expansion. However, bash 4.1 does not obey this rule.

Finally, Posix states that when mixing ‘${a=b}’ with regular
commands, it is unspecified whether the assignments affect the parent
shell environment. It is best to perform assignments independently from
commands, to avoid the problems demonstrated in this example:

Solaris /bin/sh has a frightening bug in its handling of
literal assignments. Imagine you need set a variable to a string containing
‘}’. This ‘}’ character confuses Solaris /bin/sh
when the affected variable was already set. This bug can be exercised
by running:

It seems that ‘}’ is interpreted as matching ‘${’, even
though it is enclosed in single quotes. The problem doesn't happen
using double quotes, or when using a temporary variable holding the
problematic string.

${var=expanded-value}

On Ultrix,
running

default="yu,yaa"
: ${var="$default"}

sets var to ‘M-yM-uM-,M-yM-aM-a’, i.e., the 8th bit of
each char is set. You don't observe the phenomenon using a simple
‘echo $var’ since apparently the shell resets the 8th bit when it
expands $var. Here are two means to make this shell confess its sins:

$ cat -v <<EOF
$var
EOF

and

$ set | grep '^var=' | cat -v

One classic incarnation of this bug is:

default="a b c"
: ${list="$default"}
for c in $list; do
echo $c
done

You'll get ‘a b c’ on a single line. Why? Because there are no
spaces in ‘$list’: there are ‘M- ’, i.e., spaces with the 8th
bit set, hence no IFS splitting is performed!!!

One piece of good news is that Ultrix works fine with ‘:
${list=$default}’; i.e., if you don't quote. The bad news is
then that QNX 4.25 then sets list to the last item of
default!

The portable way out consists in using a double assignment, to switch
the 8th bit twice on Ultrix:

list=${list="$default"}

...but beware of the ‘}’ bug from Solaris (see above). For safety,
use:

test "${var+set}" = set || var={value}

${#var}

${var%word}

${var%%word}

${var#word}

${var##word}

Posix requires support for these usages, but they do not work with many
traditional shells, e.g., Solaris 10 /bin/sh.

Also, pdksh 5.2.14 mishandles some word forms. For
example if ‘$1’ is ‘a/b’ and ‘$2’ is ‘a’, then
‘${1#$2}’ should yield ‘/b’, but with pdksh it
yields the empty string.

`commands`

Posix requires shells to trim all trailing newlines from command
output before substituting it, so assignments like
‘dir=`echo "$file" | tr a A`’ do not work as expected if
‘$file’ ends in a newline.

While in general it makes no sense, do not substitute a single builtin
with side effects, because Ash 0.2, trying to optimize, does not fork a
subshell to perform the command.

For instance, if you wanted to check that cd is silent, do not
use ‘test -z "`cd /`"’ because the following can happen:

$ pwd
/tmp
$ test -z "`cd /`" && pwd
/

The result of ‘foo=`exit 1`’ is left as an exercise to the reader.

The MSYS shell leaves a stray byte in the expansion of a double-quoted
command substitution of a native program, if the end of the substitution
is not aligned with the end of the double quote. This may be worked
around by inserting another pair of quotes:

Upon interrupt or SIGTERM, some shells may abort a command substitution,
replace it with a null string, and wrongly evaluate the enclosing
command before entering the trap or ending the script. This can lead to
spurious errors:

This construct is meant to replace ‘`commands`’,
and it has most of the problems listed under `commands`.

This construct can be
nested while this is impossible to do portably with back quotes.
Unfortunately it is not yet universally supported. Most notably, even recent
releases of Solaris don't support it:

If you do use ‘$(commands)’, make sure that the commands
do not start with a parenthesis, as that would cause confusion with
a different notation ‘$((expression))’ that in modern
shells is an arithmetic expression not a command. To avoid the
confusion, insert a space between the two opening parentheses.

When it is available, using arithmetic expansion provides a noticeable
speedup in script execution; but testing for support requires
eval to avoid syntax errors. The following construct is used
by AS_VAR_ARITH to provide arithmetic computation when all
arguments are provided in decimal and without a leading zero, and all
operators are properly quoted and appear as distinct arguments:

11.9 Assignments

When setting several variables in a row, be aware that the order of the
evaluation is undefined. For instance ‘foo=1 foo=2; echo $foo’
gives ‘1’ with Solaris /bin/sh, but ‘2’ with Bash.
You must use
‘;’ to enforce the order: ‘foo=1; foo=2; echo $foo’.

Don't rely on the following to find subdir/program:

PATH=subdir$PATH_SEPARATOR$PATH program

as this does not work with Zsh 3.0.6. Use something like this
instead:

(PATH=subdir$PATH_SEPARATOR$PATH; export PATH; exec program)

Don't rely on the exit status of an assignment: Ash 0.2 does not change
the status and propagates that of the last statement:

$ false || foo=bar; echo $?
1
$ false || foo=`:`; echo $?
0

and to make things even worse, QNX 4.25 just sets the exit status
to 0 in any case:

$ foo=`exit 1`; echo $?
0

To assign default values, follow this algorithm:

If the default value is a literal and does not contain any closing
brace, use:

: "${var='my literal'}"

If the default value contains no closing brace, has to be expanded, and
the variable being initialized is not intended to be IFS-split
(i.e., it's not a list), then use:

: ${var="$default"}

If the default value contains no closing brace, has to be expanded, and
the variable being initialized is intended to be IFS-split (i.e., it's a list),
then use:

var=${var="$default"}

If the default value contains a closing brace, then use:

test "${var+set}" = set || var="has a '}'"

In most cases ‘var=${var="$default"}’ is fine, but in case of
doubt, just use the last form. See Shell Substitutions, items
‘${var:-value}’ and ‘${var=value}’
for the rationale.

11.10 Parentheses in Shell Scripts

Beware of two opening parentheses in a row, as many shell
implementations treat them specially, and Posix says that a portable
script cannot use ‘((’ outside the ‘$((’ form used for shell
arithmetic. In traditional shells, ‘((cat))’ behaves like
‘(cat)’; but many shells, including
Bash and the Korn shell, treat ‘((cat))’ as an arithmetic
expression equivalent to ‘let "cat"’, and may or may not report an
error when they detect that ‘cat’ is not a number. As another
example, ‘pdksh’ 5.2.14 does not treat the following code
as a traditional shell would:

if ((true) || false); then
echo ok
fi

To work around this problem, insert a space between the two opening
parentheses. There is a similar problem and workaround with
‘$((’; see Shell Substitutions.

11.12 Special Shell Variables

Some shell variables should not be used, since they can have a deep
influence on the behavior of the shell. In order to recover a sane
behavior from the shell, some variables should be unset; M4sh takes
care of this and provides fallback values, whenever needed, to cater
for a very old /bin/sh that does not support unset.
(see Portable Shell Programming).

As a general rule, shell variable names containing a lower-case letter
are safe; you can define and use these variables without worrying about
their effect on the underlying system, and without worrying about
whether the shell changes them unexpectedly. (The exception is the
shell variable status, as described below.)

Here is a list of names that are known to cause trouble. This list is
not exhaustive, but you should be safe if you avoid the name
status and names containing only upper-case letters and
underscores.

Many shells reserve ‘$_’ for various purposes, e.g., the name of
the last command executed.

BIN_SH

In Tru64, if BIN_SH is set to xpg4, subsidiary invocations of
the standard shell conform to Posix.

CDPATH

When this variable is set it specifies a list of directories to search
when invoking cd with a relative file name that did not start
with ‘./’ or ‘../’. Posix
1003.1-2001 says that if a nonempty directory name from CDPATH
is used successfully, cd prints the resulting absolute
file name. Unfortunately this output can break idioms like
‘abs=`cd src && pwd`’ because abs receives the name twice.
Also, many shells do not conform to this part of Posix; for
example, zsh prints the result only if a directory name
other than . was chosen from CDPATH.

In practice the shells that have this problem also support
unset, so you can work around the problem as follows:

(unset CDPATH) >/dev/null 2>&1 && unset CDPATH

You can also avoid output by ensuring that your directory name is
absolute or anchored at ‘./’, as in ‘abs=`cd ./src && pwd`’.

Configure scripts use M4sh, which automatically unsets CDPATH if
possible, so you need not worry about this problem in those scripts.

CLICOLOR_FORCE

When this variable is set, some implementations of tools like
ls attempt to add color to their output via terminal escape
sequences, even when the output is not directed to a terminal, and can
thus cause spurious failures in scripts. Configure scripts use M4sh,
which automatically unsets this variable.

DUALCASE

In the MKS shell, case statements and file name generation are
case-insensitive unless DUALCASE is nonzero.
Autoconf-generated scripts export this variable when they start up.

ENV

MAIL

MAILPATH

PS1

PS2

PS4

These variables should not matter for shell scripts, since they are
supposed to affect only interactive shells. However, at least one
shell (the pre-3.0 UWIN Korn shell) gets confused about
whether it is interactive, which means that (for example) a PS1
with a side effect can unexpectedly modify ‘$?’. To work around
this bug, M4sh scripts (including configure scripts) do something
like this:

The Korn shell uses FPATH to find shell functions, so avoid
FPATH in portable scripts. FPATH is consulted after
PATH, but you still need to be wary of tests that use PATH
to find whether a command exists, since they might report the wrong
result if FPATH is also set.

GREP_OPTIONS

When this variable is set, some implementations of grep honor
these options, even if the options include direction to enable colored
output via terminal escape sequences, and the result can cause spurious
failures when the output is not directed to a terminal. Configure
scripts use M4sh, which automatically unsets this variable.

IFS

Long ago, shell scripts inherited IFS from the environment,
but this caused many problems so modern shells ignore any environment
settings for IFS.

Don't set the first character of IFS to backslash. Indeed,
Bourne shells use the first character (backslash) when joining the
components in ‘"$@"’ and some shells then reinterpret (!) the
backslash escapes, so you can end up with backspace and other strange
characters.

The proper value for IFS (in regular code, not when performing
splits) is ‘<SPC><TAB><RET>’. The first character is
especially important, as it is used to join the arguments in ‘$*’;
however, note that traditional shells, but also bash-2.04, fail to adhere
to this and join with a space anyway.

M4sh guarantees that IFS will have the default value at the
beginning of a script, and many macros within autoconf rely on this
setting. It is okay to use blocks of shell code that temporarily change
the value of IFS in order to split on another character, but
remember to restore it before expanding further macros.

Unsetting IFS instead of resetting it to the default sequence
is not suggested, since code that tries to save and restore the
variable's value will incorrectly reset it to an empty value, thus
disabling field splitting:

unset IFS
# default separators used for field splitting
save_IFS=$IFS
IFS=:
# ...
IFS=$save_IFS
# no field splitting performed

LANG

LC_ALL

LC_COLLATE

LC_CTYPE

LC_MESSAGES

LC_MONETARY

LC_NUMERIC

LC_TIME

You should set all these variables to ‘C’ because so much
configuration code assumes the C locale and Posix requires that locale
environment variables be set to ‘C’ if the C locale is desired;
configure scripts and M4sh do that for you.
Export these variables after setting them.

LANGUAGE

LANGUAGE is not specified by Posix, but it is a GNU
extension that overrides LC_ALL in some cases, so you (or M4sh)
should set it too.

LC_ADDRESS

LC_IDENTIFICATION

LC_MEASUREMENT

LC_NAME

LC_PAPER

LC_TELEPHONE

These locale environment variables are GNU extensions. They
are treated like their Posix brethren (LC_COLLATE,
etc.) as described above.

LINENO

Most modern shells provide the current line number in LINENO.
Its value is the line number of the beginning of the current command.
M4sh, and hence Autoconf, attempts to execute configure with
a shell that supports LINENO. If no such shell is available, it
attempts to implement LINENO with a Sed prepass that replaces each
instance of the string $LINENO (not followed by an alphanumeric
character) with the line's number. In M4sh scripts you should execute
AS_LINENO_PREPARE so that these workarounds are included in
your script; configure scripts do this automatically in AC_INIT.

You should not rely on LINENO within eval or shell
functions, as the behavior differs in practice. The presence of a
quoted newline within simple commands can alter which line number is
used as the starting point for $LINENO substitutions within that
command. Also, the possibility of the Sed prepass means that you should
not rely on $LINENO when quoted, when in here-documents, or when
line continuations are used. Subshells should be OK, though. In the
following example, lines 1, 9, and 14 are portable, but the other
instances of $LINENO do not have deterministic values:

In particular, note that config.status (and any other subsidiary
script created by AS_INIT_GENERATED) might report line numbers
relative to the parent script as a result of the potential Sed pass.

NULLCMD

When executing the command ‘>foo’, zsh executes
‘$NULLCMD >foo’ unless it is operating in Bourne shell
compatibility mode and the zsh version is newer
than 3.1.6-dev-18. If you are using an older zsh
and forget to set NULLCMD,
your script might be suspended waiting for data on its standard input.

options

For zsh 4.3.10, options is treated as an associative
array even after emulate sh, so it should not be used.

PATH_SEPARATOR

On DJGPP systems, the PATH_SEPARATOR environment
variable can be set to either ‘:’ or ‘;’ to control the path
separator Bash uses to set up certain environment variables (such as
PATH). You can set this variable to ‘;’ if you want
configure to use ‘;’ as a separator; this might be useful
if you plan to use non-Posix shells to execute files. See File System Conventions, for more information about PATH_SEPARATOR.

POSIXLY_CORRECT

In the GNU environment, exporting POSIXLY_CORRECT with any value
(even empty) causes programs to try harder to conform to Posix.
Autoconf does not directly manipulate this variable, but bash
ties the shell variable POSIXLY_CORRECT to whether the script is
running in Posix mode. Therefore, take care when exporting or unsetting
this variable, so as not to change whether bash is in Posix
mode.

Posix 1003.1-2001 requires that cd and
pwd must update the PWD environment variable to point
to the logical name of the current directory, but traditional shells
do not support this. This can cause confusion if one shell instance
maintains PWD but a subsidiary and different shell does not know
about PWD and executes cd; in this case PWD
points to the wrong directory. Use ‘`pwd`’ rather than
‘$PWD’.

RANDOM

Many shells provide RANDOM, a variable that returns a different
integer each time it is used. Most of the time, its value does not
change when it is not used, but on IRIX 6.5 the value changes all
the time. This can be observed by using set. It is common
practice to use $RANDOM as part of a file name, but code
shouldn't rely on $RANDOM expanding to a nonempty string.

status

This variable is an alias to ‘$?’ for zsh (at least 3.1.6),
hence read-only. Do not use it.

11.13 Shell Functions

Nowadays, it is difficult to find a shell that does not support
shell functions at all. However, some differences should be expected.

When declaring a shell function, you must include whitespace between the
‘)’ after the function name and the start of the compound
expression, to avoid upsetting ksh. While it is possible to
use any compound command, most scripts use ‘{...}’.

Inside a shell function, you should not rely on the error status of a
subshell if the last command of that subshell was exit or
trap, as this triggers bugs in zsh 4.x; while Autoconf tries to
find a shell that does not exhibit the bug, zsh might be the only shell
present on the user's machine.

Likewise, the state of ‘$?’ is not reliable when entering a shell
function. This has the effect that using a function as the first
command in a trap handler can cause problems.

DJGPP bash 2.04 has a bug in that return from a
shell function which also used a command substitution causes a
segmentation fault. To work around the issue, you can use
return from a subshell, or ‘AS_SET_STATUS’ as last command
in the execution flow of the function (see Common Shell Constructs).

Not all shells treat shell functions as simple commands impacted by
‘set -e’, for example with Solaris 10 /bin/sh:

Shell variables and functions may share the same namespace, for example
with Solaris 10 /bin/sh:

$ f () { :; }; f=; f
f: not found

For this reason, Autoconf (actually M4sh, see Programming in M4sh)
uses the prefix ‘as_fn_’ for its functions.

Handling of positional parameters and shell options varies among shells.
For example, Korn shells reset and restore trace output (‘set -x’)
and other options upon function entry and exit. Inside a function,
IRIX sh sets ‘$0’ to the function name.

It is not portable to pass temporary environment variables to shell
functions. Solaris /bin/sh does not see the variable.
Meanwhile, not all shells follow the Posix rule that the assignment must
affect the current environment in the same manner as special built-ins.

Some ancient Bourne shell variants with function support did not reset
‘$i, i >= 0’, upon function exit, so effectively the
arguments of the script were lost after the first function invocation.
It is probably not worth worrying about these shells any more.

11.14 Limitations of Shell Builtins

No, no, we are serious: some shells do have limitations! :)

You should always keep in mind that any builtin or command may support
options, and therefore differ in behavior with arguments
starting with a dash. For instance, even the innocent ‘echo "$word"’
can give unexpected results when word starts with a dash. It is
often possible to avoid this problem using ‘echo "x$word"’, taking
the ‘x’ into account later in the pipe. Many of these limitations
can be worked around using M4sh (see Programming in M4sh).

.

Use . only with regular files (use ‘test -f’). Bash
2.03, for instance, chokes on ‘. /dev/null’. Remember that
. uses PATH if its argument contains no slashes. Also,
some shells, including bash 3.2, implicitly append the current directory
to this PATH search, even though Posix forbids it. So if you want
to use . on a file foo in the current directory, you
must use ‘. ./foo’.

Not all shells gracefully handle syntax errors within a sourced file.
On one extreme, some non-interactive shells abort the entire script. On
the other, zsh 4.3.10 has a bug where it fails to react to the
syntax error.

The Unix version 7 shell did not support
negating the exit status of commands with !, and this feature
is still absent from some shells (e.g., Solaris /bin/sh).
Other shells, such as FreeBSD /bin/sh or ash, have
bugs when using !:

is therefore not portable in practice. Typically it is easy to rewrite
such code, e.g.:

cmp file1 file2 >/dev/null 2>&1 ||
echo files differ or trouble

More generally, one can always rewrite ‘! command’ as:

if command; then (exit 1); else :; fi

{...}

Bash 3.2 (and earlier versions) sometimes does not properly set
‘$?’ when failing to write redirected output of a compound command.
This problem is most commonly observed with ‘{...}’; it does
not occur with ‘(...)’. For example:

Posix requires a syntax error if a brace list has no contents. However,
not all shells obey this rule; and on shells where empty lists are
permitted, the effect on ‘$?’ is inconsistent. To avoid problems,
ensure that a brace list is never empty.

The leading ‘(’ can be omitted safely. Unfortunately, there are
contexts where unbalanced parentheses cause other problems, such as when
using a syntax-highlighting editor that searches for the balancing
counterpart, or more importantly, when using a case statement as an
underquoted argument to an Autoconf macro. See Balancing Parentheses, for tradeoffs involved in various styles of dealing with
unbalanced ‘)’.

Some shells also have problems with backslash escaping in case you do not want
to match the backslash: both a backslash and the escaped character match this
pattern. To work around this, specify the character class in a variable, so
that quote removal does not apply afterwards, and the special characters don't
have to be backslash-escaped:

Posix requires case to give an exit status of 0 if no cases
match. However, /bin/sh in Solaris 10 does not obey this
rule. Meanwhile, it is unclear whether a case that matches, but
contains no statements, must also change the exit status to 0. The M4sh
macro AS_CASE works around these inconsistencies.

Posix 1003.1-2001 requires that cd must support
the -L (“logical”) and -P (“physical”) options,
with -L being the default. However, traditional shells do
not support these options, and their cd command has the
-P behavior.

Portable scripts should assume neither option is supported, and should
assume neither behavior is the default. This can be a bit tricky,
since the Posix default behavior means that, for example,
‘ls ..’ and ‘cd ..’ may refer to different directories if
the current logical directory is a symbolic link. It is safe to use
cd dir if dir contains no .. components.
Also, Autoconf-generated scripts check for this problem when computing
variables like ac_top_srcdir (see Configuration Actions),
so it is safe to cd to these variables.

Posix states that behavior is undefined if cd is given an
explicit empty argument. Some shells do nothing, some change to the
first entry in CDPATH, some change to HOME, and some exit
the shell rather than returning an error. Unfortunately, this means
that if ‘$var’ is empty, then ‘cd "$var"’ is less predictable
than ‘cd $var’ (at least the latter is well-behaved in all shells
at changing to HOME, although this is probably not what you wanted
in a script). You should check that a directory name was supplied
before trying to change locations.

See Special Shell Variables, for portability problems involving
cd and the CDPATH environment variable.
Also please see the discussion of the pwd command.

echo

The simple echo is probably the most surprising source of
portability troubles. It is not possible to use ‘echo’ portably
unless both options and escape sequences are omitted. Don't expect any
option.

Do not use backslashes in the arguments, as there is no consensus on
their handling. For ‘echo '\n' | wc -l’, the sh of
Solaris outputs 2, but Bash and Zsh (in sh emulation mode) output 1.
The problem is truly echo: all the shells
understand ‘'\n'’ as the string composed of a backslash and an
‘n’. Within a command substitution, ‘echo 'string\c'’ will
mess up the internal state of ksh88 on AIX 6.1 so that it will print
the first character ‘s’ only, followed by a newline, and then
entirely drop the output of the next echo in a command substitution.

Because of these problems, do not pass a string containing arbitrary
characters to echo. For example, ‘echo "$foo"’ is safe
only if you know that foo's value cannot contain backslashes and
cannot start with ‘-’.

If this may not be true, printf is in general safer and
easier to use than echo and echo -n. Thus, scripts
where portability is not a major concern should use printf
'%s\n' whenever echo could fail, and similarly use
printf %s instead of echo -n. For portable shell
scripts, instead, it is suggested to use a here-document like this:

cat <<EOF
$foo
EOF

Alternatively, M4sh provides AS_ECHO and AS_ECHO_N macros
which choose between various portable implementations: ‘echo’
or ‘print’ where they work, printf if it is available,
or else other creative tricks in order to work around the above problems.

eval

The eval command is useful in limited circumstances, e.g.,
using commands like ‘eval table_$key=\$value’ and ‘eval
value=table_$key’ to simulate a hash table when the key is known to be
alphanumeric.

You should also be wary of common bugs in eval implementations.
In some shell implementations (e.g., older ash, OpenBSD 3.8
sh, pdksh v5.2.14 99/07/13.2, and zsh
4.2.5), the arguments of ‘eval’ are evaluated in a context where
‘$?’ is 0, so they exhibit behavior like this:

$ false; eval 'echo $?'
0

The correct behavior here is to output a nonzero value,
but portable scripts should not rely on this.

Note that, even though these bugs are easily avoided,
eval is tricky to use on arbitrary arguments.
It is obviously unwise to use ‘eval $cmd’ if the string value of
‘cmd’ was derived from an untrustworthy source. But even if the
string value is valid, ‘eval $cmd’ might not work as intended,
since it causes field splitting and file name expansion to occur twice,
once for the eval and once for the command itself. It is
therefore safer to use ‘eval "$cmd"’. For example, if cmd
has the value ‘cat test?.c’, ‘eval $cmd’ might expand to the
equivalent of ‘cat test;.c’ if there happens to be a file named
test;.c in the current directory; and this in turn
mistakenly attempts to invoke cat on the file test and
then execute the command .c. To avoid this problem, use
‘eval "$cmd"’ rather than ‘eval $cmd’.

However, suppose that you want to output the text of the evaluated
command just before executing it. Assuming the previous example,
‘echo "Executing: $cmd"’ outputs ‘Executing: cat test?.c’, but
this output doesn't show the user that ‘test;.c’ is the actual name
of the copied file. Conversely, ‘eval "echo Executing: $cmd"’
works on this example, but it fails with ‘cmd='cat foo >bar'’,
since it mistakenly replaces the contents of bar by the
string ‘cat foo’. No simple, general, and portable solution to
this problem is known.

exec

Posix describes several categories of shell built-ins. Special
built-ins (such as exit) must impact the environment of the
current shell, and need not be available through exec. All
other built-ins are regular, and must not propagate variable assignments
to the environment of the current shell. However, the group of regular
built-ins is further distinguished by commands that do not require a
PATH search (such as cd), in contrast to built-ins that
are offered as a more efficient version of something that must still be
found in a PATH search (such as echo). Posix is not
clear on whether exec must work with the list of 17 utilities
that are invoked without a PATH search, and many platforms lack an
executable for some of those built-ins:

$ sh -c 'exec cd /tmp'
sh: line 0: exec: cd: not found

All other built-ins that provide utilities specified by Posix must have
a counterpart executable that exists on PATH, although Posix
allows exec to use the built-in instead of the executable.
For example, contrast bash 3.2 and pdksh 5.2.14:

Some shell scripts, such as those generated by autoconf, use a
trap to clean up before exiting. If the last shell command exited with
nonzero status, the trap also exits with nonzero status so that the
invoker can tell that an error occurred.

Unfortunately, in some shells, such as Solaris /bin/sh, an exit
trap ignores the exit command's argument. In these shells, a trap
cannot determine whether it was invoked by plain exit or by
exit 1. Instead of calling exit directly, use the
AC_MSG_ERROR macro that has a workaround for this problem.

export

The builtin export dubs a shell variable environment
variable. Each update of exported variables corresponds to an update
of the environment variables. Conversely, each environment variable
received by the shell when it is launched should be imported as a shell
variable marked as exported.

Alas, many shells, such as Solaris /bin/sh,
IRIX 6.3, IRIX 5.2,
AIX 4.1.5, and Digital Unix 4.0, forget to
export the environment variables they receive. As a result,
two variables coexist: the environment variable and the shell
variable. The following code demonstrates this failure:

#!/bin/sh
echo $FOO
FOO=bar
echo $FOO
exec /bin/sh $0

when run with ‘FOO=foo’ in the environment, these shells print
alternately ‘foo’ and ‘bar’, although they should print only
‘foo’ and then a sequence of ‘bar’s.

Therefore you should export again each environment variable
that you update; the export can occur before or after the assignment.

Posix is not clear on whether the export of an undefined
variable causes the variable to be defined with the value of an empty
string, or merely marks any future definition of a variable by that name
for export. Various shells behave differently in this regard:

Posix requires export to honor assignments made as arguments,
but older shells do not support this, including /bin/sh in
Solaris 10. Portable scripts should separate assignments and exports
into different statements.

You may not leave the do on the same line as for,
since some shells improperly grok:

for arg; do
echo "$arg"
done

If you want to explicitly refer to the positional arguments, given the
‘$@’ bug (see Shell Substitutions), use:

for arg in ${1+"$@"}; do
echo "$arg"
done

But keep in mind that Zsh, even in Bourne shell emulation mode, performs
word splitting on ‘${1+"$@"}’; see Shell Substitutions,
item ‘$@’, for more.

In Solaris /bin/sh, when the list of arguments of a
for loop starts with unquoted tokens looking like
variable assignments, the loop is not executed on those tokens:

$ /bin/sh -c 'for v in a=b c=d x e=f; do echo $v; done'
x
e=f

Thankfully, quoting the assignment-like tokens, or starting the list
with other tokens (including unquoted variable expansion that results in
an assignment-like result), avoids the problem, so it is easy to work
around:

Or, especially if the else branch is short, you can use ||.
In M4sh, the AS_IF macro provides an easy way to write these kinds
of conditionals:

AS_IF([cmp -s file file.new], [], [mv file.new file])

This is especially useful in other M4 macros, where the then and
else branches might be macro arguments.

Some very old shells did not reset the exit status from an if
with no else:

$ if (exit 42); then true; fi; echo $?
42

whereas a proper shell should have printed ‘0’. But this is no
longer a portability problem; any shell that supports functions gets it
correct. However, it explains why some makefiles have lengthy
constructs:

if test -f "$file"; then
install "$file" "$dest"
else
:
fi

printf

A format string starting with a ‘-’ can cause problems.
Bash interprets it as an option and
gives an error. And ‘--’ to mark the end of options is not good
in the NetBSD Almquist shell (e.g., 0.4.6) which takes that
literally as the format string. Putting the ‘-’ in a ‘%c’
or ‘%s’ is probably easiest:

printf %s -foo

Bash 2.03 mishandles an escape sequence that happens to evaluate to ‘%’:

$ printf '\045'
bash: printf: `%': missing format character

Large outputs may cause trouble. On Solaris 2.5.1 through 10, for
example, /usr/bin/printf is buggy, so when using
/bin/sh the command ‘printf %010000x 123’ normally dumps
core.

Since printf is not always a shell builtin, there is a
potential speed penalty for using printf '%s\n' as a replacement
for an echo that does not interpret ‘\’ or leading
‘-’. With Solaris ksh, it is possible to use print
-r -- for this role instead.

With modern shells, plain pwd outputs a “logical”
directory name, some of whose components may be symbolic links. These
directory names are in contrast to “physical” directory names, whose
components are all directories.

Posix 1003.1-2001 requires that pwd must support
the -L (“logical”) and -P (“physical”) options,
with -L being the default. However, traditional shells do
not support these options, and their pwd command has the
-P behavior.

Portable scripts should assume neither option is supported, and should
assume neither behavior is the default. Also, on many hosts
‘/bin/pwd’ is equivalent to ‘pwd -P’, but Posix
does not require this behavior and portable scripts should not rely on
it.

Typically it's best to use plain pwd. On modern hosts this
outputs logical directory names, which have the following advantages:

Logical names are what the user specified.

Physical names may not be portable from one installation
host to another due to network file system gymnastics.

On modern hosts ‘pwd -P’ may fail due to lack of permissions to
some parent directory, but plain pwd cannot fail for this
reason.

Also please see the discussion of the cd command.

read

No options are portable, not even support -r (Solaris
/bin/sh for example). Tru64/OSF 5.1 sh treats
read as a special built-in, so it may exit if input is
redirected from a non-existent or unreadable file.

set

With the FreeBSD 6.0 shell, the set command (without
any options) does not sort its output.

The set builtin faces the usual problem with arguments
starting with a
dash. Modern shells such as Bash or Zsh understand -- to specify
the end of the options (any argument after -- is a parameter,
even ‘-x’ for instance), but many traditional shells (e.g., Solaris
10 /bin/sh) simply stop option
processing as soon as a non-option argument is found. Therefore, use
‘dummy’ or simply ‘x’ to end the option processing, and use
shift to pop it out:

Some nonstandard shells do not recognize more than one option
(e.g., ‘set -e -x’ assigns ‘-x’ to the command line). It is
better to combine them:

set -ex

The option -e has historically been underspecified, with enough
ambiguities to cause numerous differences across various shell
implementations; see for example
this overview,
or this link,
documenting a change to Posix 2008 to match ksh88 behavior.
Note that mixing set -e and shell functions is asking for surprises:

set -e
doit()
{
rm file
echo one
}
doit || echo two

According to the recommendation, ‘one’ should always be output
regardless of whether the rm failed, because it occurs within
the body of the shell function ‘doit’ invoked on the left side of
‘||’, where the effects of ‘set -e’ are not enforced.
Likewise, ‘two’ should never be printed, since the failure of
rm does not abort the function, such that the status of
‘doit’ is 0.

The BSD shell has had several problems with the -e
option. Older versions of the BSD
shell (circa 1990) mishandled ‘&&’, ‘||’, ‘if’, and
‘case’ when -e was in effect, causing the shell to exit
unexpectedly in some cases. This was particularly a problem with
makefiles, and led to circumlocutions like ‘sh -c 'test -f file ||
touch file'’, where the seemingly-unnecessary ‘sh -c '...'’
wrapper works around the bug (see Failure in Make Rules).

Even relatively-recent versions of the BSD shell (e.g., OpenBSD 3.4)
wrongly exit with -e if the last command within a compound
statement fails and is guarded by an ‘&&’ only. For example:

does not print ‘three’. One workaround is to change the last
instance of ‘test -n "$foo" && exit 1’ to be ‘if test -n
"$foo"; then exit 1; fi’ instead. Another possibility is to warn BSD
users not to use ‘sh -e’.

When ‘set -e’ is in effect, a failed command substitution in
Solaris /bin/sh cannot be ignored, even with ‘||’.

Portable scripts should not use ‘set -e’ if trap is used
to install an exit handler. This is because Tru64/OSF 5.1 sh
sometimes enters the trap handler with the exit status of the command
prior to the one that triggered the errexit handler:

Thus, when writing a script in M4sh, rather than trying to rely on
‘set -e’, it is better to append ‘|| AS_EXIT’ to any
statement where it is desirable to abort on failure.

Job control is not provided by all shells, so the use of ‘set -m’
or ‘set -b’ must be done with care. When using zsh in
native mode, asynchronous notification (‘set -b’) is enabled by
default, and using ‘emulate sh’ to switch to Posix mode does not
clear this setting (although asynchronous notification has no impact
unless job monitoring is also enabled). Also, zsh 4.3.10 and
earlier have a bug where job control can be manipulated in interactive
shells, but not in subshells or scripts. Furthermore, some shells, like
pdksh, fail to treat subshells as interactive, even though the
parent shell was.

Use of set -n (typically via sh -n script) to
validate a script is not foolproof. Modern ksh93 tries to be
helpful by informing you about better syntax, but switching the script
to use the suggested syntax in order to silence the warnings would
render the script no longer portable to older shells:

$ ksh -nc '``'
ksh: warning: line 1: `...` obsolete, use $(...)
0

Furthermore, on ancient hosts, such as SunOS 4, sh -n could go
into an infinite loop; even with that bug fixed, Solaris 8
/bin/sh takes extremely long to parse large scripts. Autoconf
itself uses sh -n within its testsuite to check that correct
scripts were generated, but only after first probing for other shell
features (such as test -n "${BASH_VERSION+set}") that indicate
a reasonably fast and working implementation.

shift

Not only is shifting a bad idea when there is nothing left to
shift, but in addition it is not portable: the shell of MIPS
RISC/OS 4.52 refuses to do it.

Don't use ‘shift 2’ etc.; while it in the SVR1 shell (1983),
it is also absent in many pre-Posix shells.

source

This command is not portable, as Posix does not require it; use
. instead.

test

The test program is the way to perform many file and string
tests. It is often invoked by the alternate name ‘[’, but using
that name in Autoconf code is asking for trouble since it is an M4 quote
character.

The -a, -o, ‘(’, and ‘)’ operands are not
present in all implementations, and have been marked obsolete by Posix
2008. This is because there are inherent ambiguities in using them.
For example, ‘test "$1" -a "$2"’ looks like a binary operator to
check whether two strings are both non-empty, but if ‘$1’ is the
literal ‘!’, then some implementations of test treat it
as a negation of the unary operator -a.

Thus, portable uses of test should never have more than four
arguments, and scripts should use shell constructs like ‘&&’ and
‘||’ instead. If you combine ‘&&’ and ‘||’ in the same
statement, keep in mind that they have equal precedence, so it is often
better to parenthesize even when this is redundant. For example:

test does not process options like most other commands do; for
example, it does not recognize the -- argument as marking the
end of options.

It is safe to use ‘!’ as a test operator. For example,
‘if test ! -d foo; ...’ is portable even though ‘if ! test
-d foo; ...’ is not.

test (files)

To enable configure scripts to support cross-compilation, they
shouldn't do anything that tests features of the build system instead of
the host system. But occasionally you may find it necessary to check
whether some arbitrary file exists. To do so, use ‘test -f’,
‘test -r’, or ‘test -x’. Do not use ‘test -e’, because
Solaris 10 /bin/sh
lacks it. To test for symbolic links on systems that have them, use
‘test -h’ rather than ‘test -L’; either form conforms to
Posix 1003.1-2001, but older shells like Solaris 8
/bin/sh support only -h.

For historical reasons, Posix reluctantly allows implementations of
‘test -x’ that will succeed for the root user, even if no execute
permissions are present. Furthermore, shells do not all agree on
whether Access Control Lists should affect ‘test -r’, ‘test
-w’, and ‘test -x’; some shells base test results strictly on the
current user id compared to file owner and mode, as if by
stat(2); while other shells base test results on whether the
current user has the given right, even if that right is only granted by
an ACL, as if by faccessat(2). Furthermore, there is a classic
time of check to time of use race between any use of test
followed by operating on the just-checked file. Therefore, it is a good
idea to write scripts that actually attempt an operation, and are
prepared for the resulting failure if permission is denied, rather than
trying to avoid an operation based solely on whether test
guessed that it might not be permitted.

test (strings)

Posix says that ‘test "string"’ succeeds if string is
not null, but this usage is not portable to traditional platforms like
Solaris 10 /bin/sh, which mishandle strings like ‘!’ and
‘-n’.

Posix also says that ‘test ! "string"’,
‘test -n "string"’ and
‘test -z "string"’ work with any string, but many
shells (such as Solaris, AIX 3.2, UNICOS 10.0.0.6,
Digital Unix 4, etc.) get confused if
string looks like an operator:

Similarly, Posix says that both ‘test "string1" = "string2"’
and ‘test "string1" != "string2"’ work for any pairs of
strings, but in practice this is not true for troublesome strings that
look like operators or parentheses, or that begin with ‘-’.

It is best to protect such strings with a leading ‘X’, e.g.,
‘test "Xstring" != X’ rather than ‘test -n
"string"’ or ‘test ! "string"’.

It is common to find variations of the following idiom:

test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" &&
action

to take an action when a token matches a given pattern. Such constructs
should be avoided by using:

case $ac_feature in
*[!-a-zA-Z0-9_]*) action;;
esac

If the pattern is a complicated regular expression that cannot be
expressed as a shell pattern, use something like this instead:

It is safe to trap at least the signals 1, 2, 13, and 15. You can also
trap 0, i.e., have the trap run when the script ends (either via an
explicit exit, or the end of the script). The trap for 0 should be
installed outside of a shell function, or AIX 5.3 /bin/sh
will invoke the trap at the end of this function.

Posix says that ‘trap - 1 2 13 15’ resets the traps for the
specified signals to their default values, but many common shells (e.g.,
Solaris /bin/sh) misinterpret this and attempt to execute a
“command” named - when the specified conditions arise.
Posix 2008 also added a requirement to support ‘trap 1 2 13 15’ to
reset traps, as this is supported by a larger set of shells, but there
are still shells like dash that mistakenly try to execute
1 instead of resetting the traps. Therefore, there is no
portable workaround, except for ‘trap - 0’, for which
‘trap '' 0’ is a portable substitute.

Although Posix is not absolutely clear on this point, it is widely
admitted that when entering the trap ‘$?’ should be set to the exit
status of the last command run before the trap. The ambiguity can be
summarized as: “when the trap is launched by an exit, what is
the last command run: that before exit, or
exit itself?”

Bash considers exit to be the last command, while Zsh and
Solaris /bin/sh consider that when the trap is run it is
still in the exit, hence it is the previous exit status
that the trap receives:

The portable solution is then simple: when you want to ‘exit 42’,
run ‘(exit 42); exit 42’, the first exit being used to
set the exit status to 42 for Zsh, and the second to trigger the trap
and pass 42 as exit status for Bash. In M4sh, this is covered by using
AS_EXIT.

The shell in FreeBSD 4.0 has the following bug: ‘$?’ is
reset to 0 by empty lines if the code is inside trap.

$ trap 'false
echo $?' 0
$ exit
0

Fortunately, this bug only affects trap.

Several shells fail to execute an exit trap that is defined inside a
subshell, when the last command of that subshell is not a builtin. A
workaround is to use ‘exit $?’ as the shell builtin.

Don't worry: as far as we know true is portable.
Nevertheless, it's not always a builtin (e.g., Bash 1.x), and the
portable shell community tends to prefer using :. This has a
funny side effect: when asked whether false is more portable
than true Alexandre Oliva answered:

In a sense, yes, because if it doesn't exist, the shell will produce an
exit status of failure, which is correct for false, but not
for true.

Remember that even though ‘:’ ignores its arguments, it still takes
time to compute those arguments. It is a good idea to use double quotes
around any arguments to ‘:’ to avoid time spent in field splitting
and file name expansion.

unset

In some nonconforming shells (e.g., Solaris 10 /bin/ksh and
/usr/xpg4/bin/sh, NetBSD 5.99.43 sh, or Bash 2.05a),
unset FOO fails when FOO is not set. This can interfere
with set -e operation. You can use

FOO=; unset FOO

if you are not sure that FOO is set.

A few ancient shells lack unset entirely. For some variables
such as PS1, you can use a neutralizing value instead:

PS1='$ '

Usually, shells that do not support unset need less effort to
make the environment sane, so for example is not a problem if you cannot
unset CDPATH on those shells. However, Bash 2.01 mishandles
unset MAIL and unset MAILPATH in some cases and dumps core.
So, you should do something like

Posix says that if a program contains only ‘BEGIN’ actions, and
contains no instances of getline, then the program merely
executes the actions without reading input. However, traditional Awk
implementations (such as Solaris 10 awk) read and discard
input in this case. Portable scripts can redirect input from
/dev/null to work around the problem. For example:

awk 'BEGIN {print "hello world"}' </dev/null

Posix says that in an ‘END’ action, ‘$NF’ (and presumably,
‘$1’) retain their value from the last record read, if no
intervening ‘getline’ occurred. However, some implementations
(such as Solaris 10 ‘/usr/bin/awk’, ‘nawk’, or Darwin
‘awk’) reset these variables. A workaround is to use an
intermediate variable prior to the ‘END’ block. For example:

Either do not depend on such patterns (i.e., use ‘/^(.*foo|bar)/’,
or use a simple test to reject such implementations.

On ‘ia64-hp-hpux11.23’, Awk mishandles printf conversions
after %u:

$ awk 'BEGIN { printf "%u %d\n", 0, -1 }'
0 0

AIX version 5.2 has an arbitrary limit of 399 on the
length of regular expressions and literal strings in an Awk program.

Traditional Awk implementations derived from Unix version 7, such as
Solaris /bin/awk, have many limitations and do not
conform to Posix. Nowadays AC_PROG_AWK (see Particular Programs) finds you an Awk that doesn't have these problems, but if
for some reason you prefer not to use AC_PROG_AWK you may need to
address them. For more detailed descriptions, see awk language history.

Traditional Awk does not support multidimensional arrays or user-defined
functions.

Traditional Awk does not support the -v option. You can use
assignments after the program instead, e.g., $AWK '{print v
$1}' v=x; however, don't forget that such assignments are not
evaluated until they are encountered (e.g., after any BEGIN
action).

Traditional Awk does not support the keywords delete or do.

Traditional Awk does not support the expressions
a?b:c, !a, a^b,
or a^=b.

Traditional Awk does not support the predefined CONVFMT or
ENVIRON variables.

Traditional Awk getline is not at all compatible with Posix;
avoid it.

Traditional Awk has for (i in a) ... but no other uses of the
in keyword. For example, it lacks if (i in a) ....

In code portable to both traditional and modern Awk, FS must be a
string containing just one ordinary character, and similarly for the
field-separator argument to split.

Traditional Awk has a limit of 99 fields in a record. Since some Awk
implementations, like Tru64's, split the input even if you don't refer
to any field in the script, to circumvent this problem, set ‘FS’
to an unusual character and use split.

Traditional Awk has a limit of at most 99 bytes in a number formatted by
OFMT; for example, OFMT="%.300e"; print 0.1; typically
dumps core.

The original version of Awk had a limit of at most 99 bytes per
split field, 99 bytes per substr substring, and 99 bytes
per run of non-special characters in a printf format, but these
bugs have been fixed on all practical hosts that we know of.

HP-UX 11.00 and IRIX 6.5 Awk require that input files have a line length
of at most 3070 bytes.

basename

Not all hosts have a working basename.
You can use expr instead.

cat

Don't rely on any option.

cc

The command ‘cc -c foo.c’ traditionally produces an object file
named foo.o. Most compilers allow -c to be combined
with -o to specify a different object file name, but
Posix does not require this combination and a few compilers
lack support for it. See C Compiler, for how GNU Make
tests for this feature with AC_PROG_CC_C_O.

When a compilation such as ‘cc -o foo foo.c’ fails, some compilers
(such as CDS on Reliant Unix) leave a foo.o.

The C compiler's traditional name is cc, but other names like
gcc are common. Posix 1003.1-2001 specifies the
name c99, but older Posix editions specified
c89 and anyway these standard names are rarely used in
practice. Typically the C compiler is invoked from makefiles that use
‘$(CC)’, so the value of the ‘CC’ make variable selects the
compiler name.

chgrp

chown

It is not portable to change a file's group to a group that the owner
does not belong to.

chmod

Avoid usages like ‘chmod -w file’; use ‘chmod a-w file’
instead, for two reasons. First, plain -w does not necessarily
make the file unwritable, since it does not affect mode bits that
correspond to bits in the file mode creation mask. Second,
Posix says that the -w might be interpreted as an
implementation-specific option, not as a mode; Posix suggests
using ‘chmod -- -w file’ to avoid this confusion, but unfortunately
‘--’ does not work on some older hosts.

cmp

cmp performs a raw data comparison of two files, while
diff compares two text files. Therefore, if you might compare
DOS files, even if only checking whether two files are different, use
diff to avoid spurious differences due to differences of
newline encoding.

cp

Avoid the -r option, since Posix 1003.1-2004 marks it as
obsolescent and its behavior on special files is implementation-defined.
Use -R instead. On GNU hosts the two options
are equivalent, but on Solaris hosts (for example) cp -r
reads from pipes instead of replicating them. AIX 5.3 cp -R may
corrupt its own memory with some directory hierarchies and error out or
dump core:

Some cp implementations (e.g., BSD/OS 4.2) do not allow
trailing slashes at the end of nonexistent destination directories. To
avoid this problem, omit the trailing slashes. For example, use
‘cp -R source /tmp/newdir’ rather than ‘cp -R source
/tmp/newdir/’ if /tmp/newdir does not exist.

The ancient SunOS 4 cp does not support -f, although
its mv does.

Traditionally, file timestamps had 1-second resolution, and ‘cp
-p’ copied the timestamps exactly. However, many modern file systems
have timestamps with 1-nanos